| Message ID | 20230803154453.1488248-3-gpiccoli@igalia.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Supporting same fsid mounting through a compat_ro feature | expand |
On 2023/8/3 23:43, Guilherme G. Piccoli wrote: > Btrfs doesn't currently support to mount 2 different devices holding the > same filesystem - the fsid is used as a unique identifier in the driver. > This case is supported though in some other common filesystems, like > ext4; one of the reasons for which is not trivial supporting this case > on btrfs is due to its multi-device filesystem nature, native RAID, etc. > > Supporting the same-fsid mounts has the advantage of allowing btrfs to > be used in A/B partitioned devices, like mobile phones or the Steam Deck > for example. Without this support, it's not safe for users to keep the > same "image version" in both A and B partitions, a setup that is quite > common for development, for example. Also, as a big bonus, it allows fs > integrity check based on block devices for RO devices (whereas currently > it is required that both have different fsid, breaking the block device > hash comparison). > > Such same-fsid mounting is hereby added through the usage of the > filesystem feature "single-dev" - when such feature is used, btrfs > generates a random fsid for the filesystem and leverages the long-term > present metadata_uuid infrastructure to enable the usage of this > secondary virtual fsid, effectively requiring few non-invasive changes > to the code and no new potential corner cases. My concern is still about the "virtual" fsid part. If we go virtual fsid, there can be some unexpected problems. E.g. the /sys/fs/btrfs/<uuid>/ entry would be the new virtual one. And there may be some other problems like user space UUID detection of mounted fs, thus I'm not 100% sure if this is a good idea. However I don't have any better solution either, so this may be the least worst solution for now. Thanks, Qu > > In order to prevent more code complexity and corner cases, given > the nature of this mechanism (single-devices), the single-dev feature > is not allowed when the metadata_uuid flag is already present on the > fs, or if the device is on fsid-change state. Device removal/replace > is also disabled for devices presenting the single-dev feature. > > Suggested-by: John Schoenick <johns@valvesoftware.com> > Suggested-by: Qu Wenruo <wqu@suse.com> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> > --- > fs/btrfs/disk-io.c | 19 +++++++- > fs/btrfs/fs.h | 3 +- > fs/btrfs/ioctl.c | 18 +++++++ > fs/btrfs/super.c | 8 ++-- > fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- > fs/btrfs/volumes.h | 3 +- > include/uapi/linux/btrfs.h | 7 +++ > 7 files changed, 127 insertions(+), 28 deletions(-) > > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c > index 669b10355091..455fa4949c98 100644 > --- a/fs/btrfs/disk-io.c > +++ b/fs/btrfs/disk-io.c > @@ -320,7 +320,7 @@ static bool check_tree_block_fsid(struct extent_buffer *eb) > /* > * alloc_fs_devices() copies the fsid into metadata_uuid if the > * metadata_uuid is unset in the superblock, including for a seed device. > - * So, we can use fs_devices->metadata_uuid. > + * So, we can use fs_devices->metadata_uuid; same for SINGLE_DEV devices. > */ > if (!memcmp(fsid, fs_info->fs_devices->metadata_uuid, BTRFS_FSID_SIZE)) > return false; > @@ -2288,6 +2288,7 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > { > u64 nodesize = btrfs_super_nodesize(sb); > u64 sectorsize = btrfs_super_sectorsize(sb); > + u8 *fsid; > int ret = 0; > > if (btrfs_super_magic(sb) != BTRFS_MAGIC) { > @@ -2368,7 +2369,21 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > ret = -EINVAL; > } > > - if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE)) { > + /* > + * For SINGLE_DEV devices, btrfs creates a random fsid and makes > + * use of the metadata_uuid infrastructure in order to allow, for > + * example, two devices with same fsid getting mounted at the same > + * time. But notice no changes happen at the disk level, so the > + * random generated fsid is a driver abstraction, not to be written > + * in the disk. That's the reason we're required here to compare the > + * fsid with the metadata_uuid for such devices. > + */ > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) > + fsid = fs_info->fs_devices->metadata_uuid; > + else > + fsid = fs_info->fs_devices->fsid; > + > + if (memcmp(fsid, sb->fsid, BTRFS_FSID_SIZE)) { > btrfs_err(fs_info, > "superblock fsid doesn't match fsid of fs_devices: %pU != %pU", > sb->fsid, fs_info->fs_devices->fsid); > diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h > index 203d2a267828..c6d124973361 100644 > --- a/fs/btrfs/fs.h > +++ b/fs/btrfs/fs.h > @@ -200,7 +200,8 @@ enum { > (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE | \ > BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID | \ > BTRFS_FEATURE_COMPAT_RO_VERITY | \ > - BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE) > + BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE | \ > + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV) > > #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET 0ULL > #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR 0ULL > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > index a895d105464b..56703d87def9 100644 > --- a/fs/btrfs/ioctl.c > +++ b/fs/btrfs/ioctl.c > @@ -2678,6 +2678,12 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -2744,6 +2750,12 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -3268,6 +3280,12 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info, > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) { > btrfs_err(fs_info, "device replace not supported on extent tree v2 yet"); > return -EINVAL; > diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c > index f1dd172d8d5b..ee87189b1ccd 100644 > --- a/fs/btrfs/super.c > +++ b/fs/btrfs/super.c > @@ -883,7 +883,7 @@ static int btrfs_parse_device_options(const char *options, blk_mode_t flags) > error = -ENOMEM; > goto out; > } > - device = btrfs_scan_one_device(device_name, flags); > + device = btrfs_scan_one_device(device_name, flags, true); > kfree(device_name); > if (IS_ERR(device)) { > error = PTR_ERR(device); > @@ -1478,7 +1478,7 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type, > goto error_fs_info; > } > > - device = btrfs_scan_one_device(device_name, mode); > + device = btrfs_scan_one_device(device_name, mode, true); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > error = PTR_ERR(device); > @@ -2190,7 +2190,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > switch (cmd) { > case BTRFS_IOC_SCAN_DEV: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > ret = PTR_ERR_OR_ZERO(device); > mutex_unlock(&uuid_mutex); > break; > @@ -2204,7 +2204,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > break; > case BTRFS_IOC_DEVICES_READY: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > ret = PTR_ERR(device); > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index 73753dae111a..433a490f2de8 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -681,12 +681,14 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices, > return -EINVAL; > } > > -static u8 *btrfs_sb_metadata_uuid_or_null(struct btrfs_super_block *sb) > +static u8 *btrfs_sb_metadata_uuid_single_dev(struct btrfs_super_block *sb, > + bool has_metadata_uuid, > + bool single_dev) > { > - bool has_metadata_uuid = (btrfs_super_incompat_flags(sb) & > - BTRFS_FEATURE_INCOMPAT_METADATA_UUID); > + if (has_metadata_uuid || single_dev) > + return sb->metadata_uuid; > > - return has_metadata_uuid ? sb->metadata_uuid : NULL; > + return NULL; > } > > u8 *btrfs_sb_fsid_ptr(struct btrfs_super_block *sb) > @@ -775,8 +777,36 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > > return NULL; > } > + > +static void prepare_virtual_fsid(struct btrfs_super_block *disk_super, > + const char *path) > +{ > + struct btrfs_fs_devices *fs_devices; > + u8 vfsid[BTRFS_FSID_SIZE]; > + bool dup_fsid = true; > + > + while (dup_fsid) { > + dup_fsid = false; > + generate_random_uuid(vfsid); > + > + list_for_each_entry(fs_devices, &fs_uuids, fs_list) { > + if (!memcmp(vfsid, fs_devices->fsid, BTRFS_FSID_SIZE) || > + !memcmp(vfsid, fs_devices->metadata_uuid, > + BTRFS_FSID_SIZE)) > + dup_fsid = true; > + } > + } > + > + memcpy(disk_super->metadata_uuid, disk_super->fsid, BTRFS_FSID_SIZE); > + memcpy(disk_super->fsid, vfsid, BTRFS_FSID_SIZE); > + > + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", > + disk_super->fsid, path, disk_super->metadata_uuid); > +} > + > /* > - * Add new device to list of registered devices > + * Add new device to list of registered devices, or in case of a SINGLE_DEV > + * device, also creates a virtual fsid to cope with same-fsid cases. > * > * Returns: > * device pointer which was just added or updated when successful > @@ -784,7 +814,7 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > */ > static noinline struct btrfs_device *device_list_add(const char *path, > struct btrfs_super_block *disk_super, > - bool *new_device_added) > + bool *new_device_added, bool single_dev) > { > struct btrfs_device *device; > struct btrfs_fs_devices *fs_devices = NULL; > @@ -805,23 +835,32 @@ static noinline struct btrfs_device *device_list_add(const char *path, > return ERR_PTR(error); > } > > - if (fsid_change_in_progress) { > - if (!has_metadata_uuid) > - fs_devices = find_fsid_inprogress(disk_super); > - else > - fs_devices = find_fsid_changed(disk_super); > - } else if (has_metadata_uuid) { > - fs_devices = find_fsid_with_metadata_uuid(disk_super); > + if (single_dev) { > + if (has_metadata_uuid || fsid_change_in_progress) { > + btrfs_err(NULL, > + "SINGLE_DEV devices don't support the metadata_uuid feature\n"); > + return ERR_PTR(-EINVAL); > + } > + prepare_virtual_fsid(disk_super, path); > } else { > - fs_devices = find_fsid_reverted_metadata(disk_super); > - if (!fs_devices) > - fs_devices = find_fsid(disk_super->fsid, NULL); > + if (fsid_change_in_progress) { > + if (!has_metadata_uuid) > + fs_devices = find_fsid_inprogress(disk_super); > + else > + fs_devices = find_fsid_changed(disk_super); > + } else if (has_metadata_uuid) { > + fs_devices = find_fsid_with_metadata_uuid(disk_super); > + } else { > + fs_devices = find_fsid_reverted_metadata(disk_super); > + if (!fs_devices) > + fs_devices = find_fsid(disk_super->fsid, NULL); > + } > } > > - > if (!fs_devices) { > fs_devices = alloc_fs_devices(disk_super->fsid, > - btrfs_sb_metadata_uuid_or_null(disk_super)); > + btrfs_sb_metadata_uuid_single_dev(disk_super, > + has_metadata_uuid, single_dev)); > if (IS_ERR(fs_devices)) > return ERR_CAST(fs_devices); > > @@ -1365,13 +1404,15 @@ int btrfs_forget_devices(dev_t devt) > * and we are not allowed to call set_blocksize during the scan. The superblock > * is read via pagecache > */ > -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) > +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, > + bool mounting) > { > struct btrfs_super_block *disk_super; > bool new_device_added = false; > struct btrfs_device *device = NULL; > struct block_device *bdev; > u64 bytenr, bytenr_orig; > + bool single_dev; > int ret; > > lockdep_assert_held(&uuid_mutex); > @@ -1410,7 +1451,17 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) > goto error_bdev_put; > } > > - device = device_list_add(path, disk_super, &new_device_added); > + single_dev = btrfs_super_compat_ro_flags(disk_super) & > + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV; > + > + if (!mounting && single_dev) { > + pr_info("BTRFS: skipped non-mount scan on SINGLE_DEV device %s\n", > + path); > + btrfs_release_disk_super(disk_super); > + return ERR_PTR(-EINVAL); > + } > + > + device = device_list_add(path, disk_super, &new_device_added, single_dev); > if (!IS_ERR(device) && new_device_added) > btrfs_free_stale_devices(device->devt, device); > > @@ -2406,6 +2457,12 @@ int btrfs_get_dev_args_from_path(struct btrfs_fs_info *fs_info, > > args->devid = btrfs_stack_device_id(&disk_super->dev_item); > memcpy(args->uuid, disk_super->dev_item.uuid, BTRFS_UUID_SIZE); > + > + /* > + * Note that SINGLE_DEV devices are not handled in a special way here; > + * device removal/replace is instead forbidden when such feature is > + * present, this note is for future users/readers of this function. > + */ > if (btrfs_fs_incompat(fs_info, METADATA_UUID)) > memcpy(args->fsid, disk_super->metadata_uuid, BTRFS_FSID_SIZE); > else > diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h > index 0f87057bb575..b9856c801567 100644 > --- a/fs/btrfs/volumes.h > +++ b/fs/btrfs/volumes.h > @@ -611,7 +611,8 @@ struct btrfs_block_group *btrfs_create_chunk(struct btrfs_trans_handle *trans, > void btrfs_mapping_tree_free(struct extent_map_tree *tree); > int btrfs_open_devices(struct btrfs_fs_devices *fs_devices, > blk_mode_t flags, void *holder); > -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags); > +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, > + bool mounting); > int btrfs_forget_devices(dev_t devt); > void btrfs_close_devices(struct btrfs_fs_devices *fs_devices); > void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices); > diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h > index dbb8b96da50d..cb7a7cfe1ea9 100644 > --- a/include/uapi/linux/btrfs.h > +++ b/include/uapi/linux/btrfs.h > @@ -313,6 +313,13 @@ struct btrfs_ioctl_fs_info_args { > */ > #define BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE (1ULL << 3) > > +/* > + * Single devices (as flagged by the corresponding compat_ro flag) only > + * gets scanned during mount time; also, a random fsid is generated for > + * them, in order to cope with same-fsid filesystem mounts. > + */ > +#define BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV (1ULL << 4) > + > #define BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF (1ULL << 0) > #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL (1ULL << 1) > #define BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS (1ULL << 2)
On 04/08/2023 05:27, Qu Wenruo wrote: > [...] > My concern is still about the "virtual" fsid part. > > If we go virtual fsid, there can be some unexpected problems. > > E.g. the /sys/fs/btrfs/<uuid>/ entry would be the new virtual one. > > And there may be some other problems like user space UUID detection of > mounted fs, thus I'm not 100% sure if this is a good idea. > > However I don't have any better solution either, so this may be the > least worst solution for now. > > Thanks, > Qu Hi Qu, thanks for your analysis! I think the virtual/spoofed fsid part is not without problems but I consider it to be less prone to unexpected issues than not. It's based on the metadata_uuid code, which is stable and present in btrfs for like 5 years. Also, we don't need to "corner-case" a lot of stuff to use that, which would be needed if we went to the pure dup fsid route. I tried that and it breaks in a lot of places, to which we require a lot of if conditionals (I even discussed that briefly in the last thread with you, about sysfs, remember?). So, despite not perfect, I agree with you that seems to be the least worse solution :) Cheers, Guilherme
On Thu, Aug 03, 2023 at 12:43:40PM -0300, Guilherme G. Piccoli wrote: > Btrfs doesn't currently support to mount 2 different devices holding the > same filesystem - the fsid is used as a unique identifier in the driver. > This case is supported though in some other common filesystems, like > ext4; one of the reasons for which is not trivial supporting this case > on btrfs is due to its multi-device filesystem nature, native RAID, etc. > > Supporting the same-fsid mounts has the advantage of allowing btrfs to > be used in A/B partitioned devices, like mobile phones or the Steam Deck > for example. Without this support, it's not safe for users to keep the > same "image version" in both A and B partitions, a setup that is quite > common for development, for example. Also, as a big bonus, it allows fs > integrity check based on block devices for RO devices (whereas currently > it is required that both have different fsid, breaking the block device > hash comparison). > > Such same-fsid mounting is hereby added through the usage of the > filesystem feature "single-dev" - when such feature is used, btrfs > generates a random fsid for the filesystem and leverages the long-term > present metadata_uuid infrastructure to enable the usage of this > secondary virtual fsid, effectively requiring few non-invasive changes > to the code and no new potential corner cases. > > In order to prevent more code complexity and corner cases, given > the nature of this mechanism (single-devices), the single-dev feature > is not allowed when the metadata_uuid flag is already present on the > fs, or if the device is on fsid-change state. Device removal/replace > is also disabled for devices presenting the single-dev feature. > > Suggested-by: John Schoenick <johns@valvesoftware.com> > Suggested-by: Qu Wenruo <wqu@suse.com> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> > --- > fs/btrfs/disk-io.c | 19 +++++++- > fs/btrfs/fs.h | 3 +- > fs/btrfs/ioctl.c | 18 +++++++ > fs/btrfs/super.c | 8 ++-- > fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- > fs/btrfs/volumes.h | 3 +- > include/uapi/linux/btrfs.h | 7 +++ > 7 files changed, 127 insertions(+), 28 deletions(-) > > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c > index 669b10355091..455fa4949c98 100644 > --- a/fs/btrfs/disk-io.c > +++ b/fs/btrfs/disk-io.c > @@ -320,7 +320,7 @@ static bool check_tree_block_fsid(struct extent_buffer *eb) > /* > * alloc_fs_devices() copies the fsid into metadata_uuid if the > * metadata_uuid is unset in the superblock, including for a seed device. > - * So, we can use fs_devices->metadata_uuid. > + * So, we can use fs_devices->metadata_uuid; same for SINGLE_DEV devices. > */ > if (!memcmp(fsid, fs_info->fs_devices->metadata_uuid, BTRFS_FSID_SIZE)) > return false; > @@ -2288,6 +2288,7 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > { > u64 nodesize = btrfs_super_nodesize(sb); > u64 sectorsize = btrfs_super_sectorsize(sb); > + u8 *fsid; > int ret = 0; > > if (btrfs_super_magic(sb) != BTRFS_MAGIC) { > @@ -2368,7 +2369,21 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > ret = -EINVAL; > } > > - if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE)) { > + /* > + * For SINGLE_DEV devices, btrfs creates a random fsid and makes > + * use of the metadata_uuid infrastructure in order to allow, for > + * example, two devices with same fsid getting mounted at the same > + * time. But notice no changes happen at the disk level, so the > + * random generated fsid is a driver abstraction, not to be written > + * in the disk. That's the reason we're required here to compare the > + * fsid with the metadata_uuid for such devices. > + */ > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) > + fsid = fs_info->fs_devices->metadata_uuid; > + else > + fsid = fs_info->fs_devices->fsid; > + > + if (memcmp(fsid, sb->fsid, BTRFS_FSID_SIZE)) { > btrfs_err(fs_info, > "superblock fsid doesn't match fsid of fs_devices: %pU != %pU", > sb->fsid, fs_info->fs_devices->fsid); > diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h > index 203d2a267828..c6d124973361 100644 > --- a/fs/btrfs/fs.h > +++ b/fs/btrfs/fs.h > @@ -200,7 +200,8 @@ enum { > (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE | \ > BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID | \ > BTRFS_FEATURE_COMPAT_RO_VERITY | \ > - BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE) > + BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE | \ > + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV) > > #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET 0ULL > #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR 0ULL > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > index a895d105464b..56703d87def9 100644 > --- a/fs/btrfs/ioctl.c > +++ b/fs/btrfs/ioctl.c > @@ -2678,6 +2678,12 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -2744,6 +2750,12 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -3268,6 +3280,12 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info, > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) { > btrfs_err(fs_info, "device replace not supported on extent tree v2 yet"); > return -EINVAL; > diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c > index f1dd172d8d5b..ee87189b1ccd 100644 > --- a/fs/btrfs/super.c > +++ b/fs/btrfs/super.c > @@ -883,7 +883,7 @@ static int btrfs_parse_device_options(const char *options, blk_mode_t flags) > error = -ENOMEM; > goto out; > } > - device = btrfs_scan_one_device(device_name, flags); > + device = btrfs_scan_one_device(device_name, flags, true); > kfree(device_name); > if (IS_ERR(device)) { > error = PTR_ERR(device); > @@ -1478,7 +1478,7 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type, > goto error_fs_info; > } > > - device = btrfs_scan_one_device(device_name, mode); > + device = btrfs_scan_one_device(device_name, mode, true); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > error = PTR_ERR(device); > @@ -2190,7 +2190,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > switch (cmd) { > case BTRFS_IOC_SCAN_DEV: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > ret = PTR_ERR_OR_ZERO(device); > mutex_unlock(&uuid_mutex); > break; > @@ -2204,7 +2204,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > break; > case BTRFS_IOC_DEVICES_READY: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > ret = PTR_ERR(device); > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index 73753dae111a..433a490f2de8 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -681,12 +681,14 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices, > return -EINVAL; > } > > -static u8 *btrfs_sb_metadata_uuid_or_null(struct btrfs_super_block *sb) > +static u8 *btrfs_sb_metadata_uuid_single_dev(struct btrfs_super_block *sb, > + bool has_metadata_uuid, > + bool single_dev) Why pass as an argument? Just do the same thing as the code currently does and check for the single device ro compat flag. > { > - bool has_metadata_uuid = (btrfs_super_incompat_flags(sb) & > - BTRFS_FEATURE_INCOMPAT_METADATA_UUID); > + if (has_metadata_uuid || single_dev) > + return sb->metadata_uuid; > > - return has_metadata_uuid ? sb->metadata_uuid : NULL; > + return NULL; > } > > u8 *btrfs_sb_fsid_ptr(struct btrfs_super_block *sb) > @@ -775,8 +777,36 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > > return NULL; > } > + > +static void prepare_virtual_fsid(struct btrfs_super_block *disk_super, > + const char *path) > +{ > + struct btrfs_fs_devices *fs_devices; > + u8 vfsid[BTRFS_FSID_SIZE]; > + bool dup_fsid = true; > + > + while (dup_fsid) { > + dup_fsid = false; > + generate_random_uuid(vfsid); > + > + list_for_each_entry(fs_devices, &fs_uuids, fs_list) { > + if (!memcmp(vfsid, fs_devices->fsid, BTRFS_FSID_SIZE) || > + !memcmp(vfsid, fs_devices->metadata_uuid, > + BTRFS_FSID_SIZE)) > + dup_fsid = true; > + } > + } > + > + memcpy(disk_super->metadata_uuid, disk_super->fsid, BTRFS_FSID_SIZE); > + memcpy(disk_super->fsid, vfsid, BTRFS_FSID_SIZE); > + > + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", > + disk_super->fsid, path, disk_super->metadata_uuid); I think just btrfs_info(NULL, "virtual fsid....") is fine here. > +} > + > /* > - * Add new device to list of registered devices > + * Add new device to list of registered devices, or in case of a SINGLE_DEV > + * device, also creates a virtual fsid to cope with same-fsid cases. > * > * Returns: > * device pointer which was just added or updated when successful > @@ -784,7 +814,7 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > */ > static noinline struct btrfs_device *device_list_add(const char *path, > struct btrfs_super_block *disk_super, > - bool *new_device_added) > + bool *new_device_added, bool single_dev) Same as the comment above. Generally speaking for stuff like this where we can derive the value local to the function we want to do that instead of growing the argument list. Thanks, Josef
On 17/08/2023 12:41, Josef Bacik wrote: >> [...] >> + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", >> + disk_super->fsid, path, disk_super->metadata_uuid); > > I think just > > btrfs_info(NULL, "virtual fsid....") > > is fine here. > So just for my full understanding, do you think we shouldn't show the real fsid here, but keep showing the virtual one, right? Or you prefer we literally show "virtual fsid...."? >> +} >> + >> /* >> - * Add new device to list of registered devices >> + * Add new device to list of registered devices, or in case of a SINGLE_DEV >> + * device, also creates a virtual fsid to cope with same-fsid cases. >> * >> * Returns: >> * device pointer which was just added or updated when successful >> @@ -784,7 +814,7 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( >> */ >> static noinline struct btrfs_device *device_list_add(const char *path, >> struct btrfs_super_block *disk_super, >> - bool *new_device_added) >> + bool *new_device_added, bool single_dev) > > Same as the comment above. Generally speaking for stuff like this where we can > derive the value local to the function we want to do that instead of growing the > argument list. Thanks, > > Josef > OK, will do it both here and above as you suggested. Thanks for the review! Cheers, Guilherme
On Thu, Aug 17, 2023 at 01:20:55PM -0300, Guilherme G. Piccoli wrote: > On 17/08/2023 12:41, Josef Bacik wrote: > >> [...] > >> + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", > >> + disk_super->fsid, path, disk_super->metadata_uuid); > > > > I think just > > > > btrfs_info(NULL, "virtual fsid....") > > > > is fine here. > > > > So just for my full understanding, do you think we shouldn't show the > real fsid here, but keep showing the virtual one, right? Or you prefer > we literally show "virtual fsid...."? Oh no sorry, just swap pr_info for btrfs_info, and keep the rest the same, so btrfs_info("virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", disk_super->fsid, path, disk_super->metadata_uuid); thanks, Josef
On 17/08/2023 13:58, Josef Bacik wrote: > [...] > Oh no sorry, just swap pr_info for btrfs_info, and keep the rest the same, so > > btrfs_info("virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", > disk_super->fsid, path, disk_super->metadata_uuid); > > thanks, > > Josef > Oh OK, thanks! My apologies, I confused the stuff heh Cheers, Guilherme
On 8/3/23 23:43, Guilherme G. Piccoli wrote: > Btrfs doesn't currently support to mount 2 different devices holding the > same filesystem - the fsid is used as a unique identifier in the driver. fsid is for the external frontend, systemd, and udev stuff; metadata_uuid pertains to the actual btrfs on-disk. > This case is supported though in some other common filesystems, like > ext4; > one of the reasons for which is not trivial supporting this case > on btrfs is due to its multi-device filesystem nature, native RAID, etc. How is it related to the multi-device aspect? The main limitation is that a disk image can be cloned without maintaining a unique device- uuid. > Supporting the same-fsid mounts has the advantage of allowing btrfs to > be used in A/B partitioned devices, like mobile phones or the Steam Deck > for example. > Without this support, it's not safe for users to keep the > same "image version" in both A and B partitions, a setup that is quite > common for development, for example. Also, as a big bonus, it allows fs > integrity check based on block devices for RO devices (whereas currently > it is required that both have different fsid, breaking the block device > hash comparison). Does it apply to smaller disk images? Otherwise, it will be very time-consuming. Just curious, how is the checksum verified for the entire disk? (Btrfs might provide a checksum tree-based solution at some point.) > Such same-fsid mounting is hereby added through the usage of the > filesystem feature "single-dev" - when such feature is used, btrfs > generates a random fsid for the filesystem and leverages the long-term > present metadata_uuid infrastructure to enable the usage of this > secondary virtual fsid, effectively requiring few non-invasive changes > to the code and no new potential corner cases. > > In order to prevent more code complexity and corner cases, given > the nature of this mechanism (single-devices), the single-dev feature > is not allowed when the metadata_uuid flag is already present on the > fs, or if the device is on fsid-change state. Device removal/replace > is also disabled for devices presenting the single-dev feature. > > Suggested-by: John Schoenick <johns@valvesoftware.com> > Suggested-by: Qu Wenruo <wqu@suse.com> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> > --- > fs/btrfs/disk-io.c | 19 +++++++- > fs/btrfs/fs.h | 3 +- > fs/btrfs/ioctl.c | 18 +++++++ > fs/btrfs/super.c | 8 ++-- > fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- > fs/btrfs/volumes.h | 3 +- > include/uapi/linux/btrfs.h | 7 +++ > 7 files changed, 127 insertions(+), 28 deletions(-) > > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c > index 669b10355091..455fa4949c98 100644 > --- a/fs/btrfs/disk-io.c > +++ b/fs/btrfs/disk-io.c > @@ -320,7 +320,7 @@ static bool check_tree_block_fsid(struct extent_buffer *eb) > /* > * alloc_fs_devices() copies the fsid into metadata_uuid if the > * metadata_uuid is unset in the superblock, including for a seed device. > - * So, we can use fs_devices->metadata_uuid. > + * So, we can use fs_devices->metadata_uuid; same for SINGLE_DEV devices. > */ > if (!memcmp(fsid, fs_info->fs_devices->metadata_uuid, BTRFS_FSID_SIZE)) > return false; > @@ -2288,6 +2288,7 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > { > u64 nodesize = btrfs_super_nodesize(sb); > u64 sectorsize = btrfs_super_sectorsize(sb); > + u8 *fsid; > int ret = 0; > > if (btrfs_super_magic(sb) != BTRFS_MAGIC) { > @@ -2368,7 +2369,21 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, > ret = -EINVAL; > } > > - if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE)) { > + /* > + * For SINGLE_DEV devices, btrfs creates a random fsid and makes > + * use of the metadata_uuid infrastructure in order to allow, for > + * example, two devices with same fsid getting mounted at the same > + * time. But notice no changes happen at the disk level, so the > + * random generated fsid is a driver abstraction, not to be written > + * in the disk. That's the reason we're required here to compare the > + * fsid with the metadata_uuid for such devices. > + */ > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) > + fsid = fs_info->fs_devices->metadata_uuid; > + else > + fsid = fs_info->fs_devices->fsid; Below alloc_fs_device(), fsid is still being kept equal to metadata_uuid in memory for single_dev. So, this distinction is unnecessary. > + > + if (memcmp(fsid, sb->fsid, BTRFS_FSID_SIZE)) { David prefers memcmp to be either compared to == or != to 0 depending on the requirement. > btrfs_err(fs_info, > "superblock fsid doesn't match fsid of fs_devices: %pU != %pU", > sb->fsid, fs_info->fs_devices->fsid); > diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h > index 203d2a267828..c6d124973361 100644 > --- a/fs/btrfs/fs.h > +++ b/fs/btrfs/fs.h > @@ -200,7 +200,8 @@ enum { > (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE | \ > BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID | \ > BTRFS_FEATURE_COMPAT_RO_VERITY | \ > - BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE) > + BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE | \ > + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV) > > #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET 0ULL > #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR 0ULL > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c > index a895d105464b..56703d87def9 100644 > --- a/fs/btrfs/ioctl.c > +++ b/fs/btrfs/ioctl.c > @@ -2678,6 +2678,12 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -2744,6 +2750,12 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg) > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > vol_args = memdup_user(arg, sizeof(*vol_args)); > if (IS_ERR(vol_args)) > return PTR_ERR(vol_args); > @@ -3268,6 +3280,12 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info, > if (!capable(CAP_SYS_ADMIN)) > return -EPERM; > > + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { > + btrfs_err(fs_info, > + "device removal is unsupported on SINGLE_DEV devices\n"); > + return -EINVAL; > + } > + > if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) { > btrfs_err(fs_info, "device replace not supported on extent tree v2 yet"); > return -EINVAL; > diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c > index f1dd172d8d5b..ee87189b1ccd 100644 > --- a/fs/btrfs/super.c > +++ b/fs/btrfs/super.c > @@ -883,7 +883,7 @@ static int btrfs_parse_device_options(const char *options, blk_mode_t flags) > error = -ENOMEM; > goto out; > } > - device = btrfs_scan_one_device(device_name, flags); > + device = btrfs_scan_one_device(device_name, flags, true); > kfree(device_name); > if (IS_ERR(device)) { > error = PTR_ERR(device); > @@ -1478,7 +1478,7 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type, > goto error_fs_info; > } > > - device = btrfs_scan_one_device(device_name, mode); > + device = btrfs_scan_one_device(device_name, mode, true); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > error = PTR_ERR(device); > @@ -2190,7 +2190,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > switch (cmd) { > case BTRFS_IOC_SCAN_DEV: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > ret = PTR_ERR_OR_ZERO(device); > mutex_unlock(&uuid_mutex); > break; > @@ -2204,7 +2204,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, > break; > case BTRFS_IOC_DEVICES_READY: > mutex_lock(&uuid_mutex); > - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); > + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); > if (IS_ERR(device)) { > mutex_unlock(&uuid_mutex); > ret = PTR_ERR(device); > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index 73753dae111a..433a490f2de8 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -681,12 +681,14 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices, > return -EINVAL; > } > > -static u8 *btrfs_sb_metadata_uuid_or_null(struct btrfs_super_block *sb) > +static u8 *btrfs_sb_metadata_uuid_single_dev(struct btrfs_super_block *sb, > + bool has_metadata_uuid, > + bool single_dev) > { > - bool has_metadata_uuid = (btrfs_super_incompat_flags(sb) & > - BTRFS_FEATURE_INCOMPAT_METADATA_UUID); > + if (has_metadata_uuid || single_dev) > + return sb->metadata_uuid; > > - return has_metadata_uuid ? sb->metadata_uuid : NULL; > + return NULL; > } > > u8 *btrfs_sb_fsid_ptr(struct btrfs_super_block *sb) You can rebase the code onto the latest misc-next branch. This is because we have dropped the function btrfs_sb_metadata_uuid_or_null() in the final integration. > @@ -775,8 +777,36 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > > return NULL; > } > + > +static void prepare_virtual_fsid(struct btrfs_super_block *disk_super, > + const char *path) > +{ > + struct btrfs_fs_devices *fs_devices; > + u8 vfsid[BTRFS_FSID_SIZE]; > + bool dup_fsid = true; > + > + while (dup_fsid) { > + dup_fsid = false; > + generate_random_uuid(vfsid); > + > + list_for_each_entry(fs_devices, &fs_uuids, fs_list) { > + if (!memcmp(vfsid, fs_devices->fsid, BTRFS_FSID_SIZE) || > + !memcmp(vfsid, fs_devices->metadata_uuid, > + BTRFS_FSID_SIZE)) > + dup_fsid = true; > + } > + } > + > + memcpy(disk_super->metadata_uuid, disk_super->fsid, BTRFS_FSID_SIZE); > + memcpy(disk_super->fsid, vfsid, BTRFS_FSID_SIZE); > + > + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", > + disk_super->fsid, path, disk_super->metadata_uuid); > +} > + > /* > - * Add new device to list of registered devices > + * Add new device to list of registered devices, or in case of a SINGLE_DEV > + * device, also creates a virtual fsid to cope with same-fsid cases. > * > * Returns: > * device pointer which was just added or updated when successful > @@ -784,7 +814,7 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( > */ > static noinline struct btrfs_device *device_list_add(const char *path, > struct btrfs_super_block *disk_super, > - bool *new_device_added) > + bool *new_device_added, bool single_dev) > { > struct btrfs_device *device; > struct btrfs_fs_devices *fs_devices = NULL; > @@ -805,23 +835,32 @@ static noinline struct btrfs_device *device_list_add(const char *path, > return ERR_PTR(error); > } > > - if (fsid_change_in_progress) { > - if (!has_metadata_uuid) > - fs_devices = find_fsid_inprogress(disk_super); > - else > - fs_devices = find_fsid_changed(disk_super); > - } else if (has_metadata_uuid) { > - fs_devices = find_fsid_with_metadata_uuid(disk_super); > + if (single_dev) { > + if (has_metadata_uuid || fsid_change_in_progress) { > + btrfs_err(NULL, > + "SINGLE_DEV devices don't support the metadata_uuid feature\n"); > + return ERR_PTR(-EINVAL); > + } > + prepare_virtual_fsid(disk_super, path); > } else { > - fs_devices = find_fsid_reverted_metadata(disk_super); > - if (!fs_devices) > - fs_devices = find_fsid(disk_super->fsid, NULL); > + if (fsid_change_in_progress) { > + if (!has_metadata_uuid) > + fs_devices = find_fsid_inprogress(disk_super); > + else > + fs_devices = find_fsid_changed(disk_super); > + } else if (has_metadata_uuid) { > + fs_devices = find_fsid_with_metadata_uuid(disk_super); > + } else { > + fs_devices = find_fsid_reverted_metadata(disk_super); > + if (!fs_devices) > + fs_devices = find_fsid(disk_super->fsid, NULL); > + } > } > > - > if (!fs_devices) { > fs_devices = alloc_fs_devices(disk_super->fsid, > - btrfs_sb_metadata_uuid_or_null(disk_super)); > + btrfs_sb_metadata_uuid_single_dev(disk_super, > + has_metadata_uuid, single_dev)); I think it is a good idea to rebase on latest misc-next and add the below patch, as the arguments of alloc_fs_device() have been simplified. [PATCH resend] btrfs: simplify alloc_fs_devices() remove arg2 Thanks, Anand > if (IS_ERR(fs_devices)) > return ERR_CAST(fs_devices); > > @@ -1365,13 +1404,15 @@ int btrfs_forget_devices(dev_t devt) > * and we are not allowed to call set_blocksize during the scan. The superblock > * is read via pagecache > */ > -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) > +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, > + bool mounting) > { > struct btrfs_super_block *disk_super; > bool new_device_added = false; > struct btrfs_device *device = NULL; > struct block_device *bdev; > u64 bytenr, bytenr_orig; > + bool single_dev; > int ret; > > lockdep_assert_held(&uuid_mutex); > @@ -1410,7 +1451,17 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) > goto error_bdev_put; > } > > - device = device_list_add(path, disk_super, &new_device_added); > + single_dev = btrfs_super_compat_ro_flags(disk_super) & > + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV; > + > + if (!mounting && single_dev) { > + pr_info("BTRFS: skipped non-mount scan on SINGLE_DEV device %s\n", > + path); > + btrfs_release_disk_super(disk_super); > + return ERR_PTR(-EINVAL); > + } > + > + device = device_list_add(path, disk_super, &new_device_added, single_dev); > if (!IS_ERR(device) && new_device_added) > btrfs_free_stale_devices(device->devt, device); > > @@ -2406,6 +2457,12 @@ int btrfs_get_dev_args_from_path(struct btrfs_fs_info *fs_info, > > args->devid = btrfs_stack_device_id(&disk_super->dev_item); > memcpy(args->uuid, disk_super->dev_item.uuid, BTRFS_UUID_SIZE); > + > + /* > + * Note that SINGLE_DEV devices are not handled in a special way here; > + * device removal/replace is instead forbidden when such feature is > + * present, this note is for future users/readers of this function. > + */ > if (btrfs_fs_incompat(fs_info, METADATA_UUID)) > memcpy(args->fsid, disk_super->metadata_uuid, BTRFS_FSID_SIZE); > else > diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h > index 0f87057bb575..b9856c801567 100644 > --- a/fs/btrfs/volumes.h > +++ b/fs/btrfs/volumes.h > @@ -611,7 +611,8 @@ struct btrfs_block_group *btrfs_create_chunk(struct btrfs_trans_handle *trans, > void btrfs_mapping_tree_free(struct extent_map_tree *tree); > int btrfs_open_devices(struct btrfs_fs_devices *fs_devices, > blk_mode_t flags, void *holder); > -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags); > +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, > + bool mounting); > int btrfs_forget_devices(dev_t devt); > void btrfs_close_devices(struct btrfs_fs_devices *fs_devices); > void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices); > diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h > index dbb8b96da50d..cb7a7cfe1ea9 100644 > --- a/include/uapi/linux/btrfs.h > +++ b/include/uapi/linux/btrfs.h > @@ -313,6 +313,13 @@ struct btrfs_ioctl_fs_info_args { > */ > #define BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE (1ULL << 3) > > +/* > + * Single devices (as flagged by the corresponding compat_ro flag) only > + * gets scanned during mount time; also, a random fsid is generated for > + * them, in order to cope with same-fsid filesystem mounts. > + */ > +#define BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV (1ULL << 4) > + > #define BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF (1ULL << 0) > #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL (1ULL << 1) > #define BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS (1ULL << 2)
Hi Anand, first of all, thanks for the review! More comments inline below. On 23/08/2023 13:31, Anand Jain wrote: > [...] > On 8/3/23 23:43, Guilherme G. Piccoli wrote: >> Btrfs doesn't currently support to mount 2 different devices holding the >> same filesystem - the fsid is used as a unique identifier in the driver. > > fsid is for the external frontend, systemd, and udev stuff; > metadata_uuid pertains to the actual btrfs on-disk. > True, agreed - but I guess my phrase is not wrong per se, right? It's more like an "abstraction" of the concept...I'm in fact talking about the fsid as the exposed entity to the system, like for udev, etc. I guess I'll keep it on V3 if you don't oppose. > [...] >> one of the reasons for which is not trivial supporting this case >> on btrfs is due to its multi-device filesystem nature, native RAID, etc. > > How is it related to the multi-device aspect? The main limitation is > that a disk image can be cloned without maintaining a unique device- > uuid. > Oh okay, I'll rephrase / drop this part, you're right - thanks. > [...] >> Without this support, it's not safe for users to keep the >> same "image version" in both A and B partitions, a setup that is quite >> common for development, for example. Also, as a big bonus, it allows fs >> integrity check based on block devices for RO devices (whereas currently >> it is required that both have different fsid, breaking the block device >> hash comparison). > > Does it apply to smaller disk images? Otherwise, it will be very > time-consuming. Just curious, how is the checksum verified for the entire > disk? (Btrfs might provide a checksum tree-based solution at > some point.) > The disk image is currently 5G, NVMe device - it's usually fast in our case. After discussing internally with the folks knowledgeable about the update process, it seems the Desync tool is responsible for that, taking an index chunk-comparison during the update. Based in my quick check, seems the codes related to that are here: https://github.com/folbricht/desync/blob/master/verifyindex.go https://github.com/folbricht/desync/blob/master/index.go >> + */ >> + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) >> + fsid = fs_info->fs_devices->metadata_uuid; >> + else >> + fsid = fs_info->fs_devices->fsid; > > Below alloc_fs_device(), fsid is still being kept equal to metadata_uuid > in memory for single_dev. So, this distinction is unnecessary. > > >> + >> + if (memcmp(fsid, sb->fsid, BTRFS_FSID_SIZE)) { > > David prefers memcmp to be either compared to == or != to 0 > depending on the requirement. > OK, thanks - I'll experiment dropping this part, and I see you have code present in for-next, changing this specific memcmp. So hopefully don't need my code anymore but if we do, I'll keep the convention =) > [...] >> -static u8 *btrfs_sb_metadata_uuid_or_null(struct btrfs_super_block *sb) >> +static u8 *btrfs_sb_metadata_uuid_single_dev(struct btrfs_super_block *sb, >> + bool has_metadata_uuid, >> + bool single_dev) >> { >> - bool has_metadata_uuid = (btrfs_super_incompat_flags(sb) & >> - BTRFS_FEATURE_INCOMPAT_METADATA_UUID); >> + if (has_metadata_uuid || single_dev) >> + return sb->metadata_uuid; >> >> - return has_metadata_uuid ? sb->metadata_uuid : NULL; >> + return NULL; >> } >> >> u8 *btrfs_sb_fsid_ptr(struct btrfs_super_block *sb) > > You can rebase the code onto the latest misc-next branch. > This is because we have dropped the function > btrfs_sb_metadata_uuid_or_null() in the final integration. > [...] >> - btrfs_sb_metadata_uuid_or_null(disk_super)); >> + btrfs_sb_metadata_uuid_single_dev(disk_super, >> + has_metadata_uuid, single_dev)); > > I think it is a good idea to rebase on latest misc-next and add the > below patch, as the arguments of alloc_fs_device() have been simplified. > > [PATCH resend] btrfs: simplify alloc_fs_devices() remove arg2 > > Very good idea, thanks for pointing that, will do it for V3! Cheers, Guilherme
On 03/08/2023 12:43, Guilherme G. Piccoli wrote: > [...] > fs/btrfs/disk-io.c | 19 +++++++- > fs/btrfs/fs.h | 3 +- > fs/btrfs/ioctl.c | 18 +++++++ > fs/btrfs/super.c | 8 ++-- > fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- > fs/btrfs/volumes.h | 3 +- > include/uapi/linux/btrfs.h | 7 +++ > 7 files changed, 127 insertions(+), 28 deletions(-) > Hi folks, while working the xfstests for this case, I've noticed the single-dev feature is not exposed in sysfs! Should we make it available there? A quick change here made me see it there, but it sticks to value 0 ... maybe I'm not really aware of how the sysfs/features directory should work, hence I appreciate if you could enlighten me if makes sense to have this feature there (and if it's OK showing zero or should flip in case a device makes use of the feature, maybe?). Thanks in advance, Guilherme
On 30/08/2023 04:28, Guilherme G. Piccoli wrote: > On 03/08/2023 12:43, Guilherme G. Piccoli wrote: >> [...] >> fs/btrfs/disk-io.c | 19 +++++++- >> fs/btrfs/fs.h | 3 +- >> fs/btrfs/ioctl.c | 18 +++++++ >> fs/btrfs/super.c | 8 ++-- >> fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- >> fs/btrfs/volumes.h | 3 +- >> include/uapi/linux/btrfs.h | 7 +++ >> 7 files changed, 127 insertions(+), 28 deletions(-) >> > > Hi folks, while working the xfstests for this case, I've noticed the > single-dev feature is not exposed in sysfs! Should we make it available > there? > > A quick change here made me see it there, but it sticks to value 0 ... > maybe I'm not really aware of how the sysfs/features directory should > work, hence I appreciate if you could enlighten me if makes sense to > have this feature there (and if it's OK showing zero or should flip in > case a device makes use of the feature, maybe?). > Yeah, we need sysfs entries for the new feature and test cases that generally rely on it to determine whether to skip or run the test case. The paths would be: - /sys/fs/btrfs/features/.. - /sys/fs/btrfs/<uuid>/features/.. These paths emit only output 0. The presence of the sysfs file indicates that the feature is supported. Thanks, Anand > Thanks in advance, > > > Guilherme
On 30/08/2023 04:11, Anand Jain wrote: > [...] > Yeah, we need sysfs entries for the new feature and test cases that > generally rely on it to determine whether to skip or run the test case. > > The paths would be: > - /sys/fs/btrfs/features/.. > - /sys/fs/btrfs/<uuid>/features/.. > > These paths emit only output 0. The presence of the sysfs file indicates > that the feature is supported. > > Thanks, > Anand > Thanks a lot Anand!
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 669b10355091..455fa4949c98 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -320,7 +320,7 @@ static bool check_tree_block_fsid(struct extent_buffer *eb) /* * alloc_fs_devices() copies the fsid into metadata_uuid if the * metadata_uuid is unset in the superblock, including for a seed device. - * So, we can use fs_devices->metadata_uuid. + * So, we can use fs_devices->metadata_uuid; same for SINGLE_DEV devices. */ if (!memcmp(fsid, fs_info->fs_devices->metadata_uuid, BTRFS_FSID_SIZE)) return false; @@ -2288,6 +2288,7 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, { u64 nodesize = btrfs_super_nodesize(sb); u64 sectorsize = btrfs_super_sectorsize(sb); + u8 *fsid; int ret = 0; if (btrfs_super_magic(sb) != BTRFS_MAGIC) { @@ -2368,7 +2369,21 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, ret = -EINVAL; } - if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE)) { + /* + * For SINGLE_DEV devices, btrfs creates a random fsid and makes + * use of the metadata_uuid infrastructure in order to allow, for + * example, two devices with same fsid getting mounted at the same + * time. But notice no changes happen at the disk level, so the + * random generated fsid is a driver abstraction, not to be written + * in the disk. That's the reason we're required here to compare the + * fsid with the metadata_uuid for such devices. + */ + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) + fsid = fs_info->fs_devices->metadata_uuid; + else + fsid = fs_info->fs_devices->fsid; + + if (memcmp(fsid, sb->fsid, BTRFS_FSID_SIZE)) { btrfs_err(fs_info, "superblock fsid doesn't match fsid of fs_devices: %pU != %pU", sb->fsid, fs_info->fs_devices->fsid); diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h index 203d2a267828..c6d124973361 100644 --- a/fs/btrfs/fs.h +++ b/fs/btrfs/fs.h @@ -200,7 +200,8 @@ enum { (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE | \ BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID | \ BTRFS_FEATURE_COMPAT_RO_VERITY | \ - BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE) + BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE | \ + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV) #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET 0ULL #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR 0ULL diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index a895d105464b..56703d87def9 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -2678,6 +2678,12 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg) if (!capable(CAP_SYS_ADMIN)) return -EPERM; + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { + btrfs_err(fs_info, + "device removal is unsupported on SINGLE_DEV devices\n"); + return -EINVAL; + } + vol_args = memdup_user(arg, sizeof(*vol_args)); if (IS_ERR(vol_args)) return PTR_ERR(vol_args); @@ -2744,6 +2750,12 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg) if (!capable(CAP_SYS_ADMIN)) return -EPERM; + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { + btrfs_err(fs_info, + "device removal is unsupported on SINGLE_DEV devices\n"); + return -EINVAL; + } + vol_args = memdup_user(arg, sizeof(*vol_args)); if (IS_ERR(vol_args)) return PTR_ERR(vol_args); @@ -3268,6 +3280,12 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info, if (!capable(CAP_SYS_ADMIN)) return -EPERM; + if (btrfs_fs_compat_ro(fs_info, SINGLE_DEV)) { + btrfs_err(fs_info, + "device removal is unsupported on SINGLE_DEV devices\n"); + return -EINVAL; + } + if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) { btrfs_err(fs_info, "device replace not supported on extent tree v2 yet"); return -EINVAL; diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index f1dd172d8d5b..ee87189b1ccd 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -883,7 +883,7 @@ static int btrfs_parse_device_options(const char *options, blk_mode_t flags) error = -ENOMEM; goto out; } - device = btrfs_scan_one_device(device_name, flags); + device = btrfs_scan_one_device(device_name, flags, true); kfree(device_name); if (IS_ERR(device)) { error = PTR_ERR(device); @@ -1478,7 +1478,7 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type, goto error_fs_info; } - device = btrfs_scan_one_device(device_name, mode); + device = btrfs_scan_one_device(device_name, mode, true); if (IS_ERR(device)) { mutex_unlock(&uuid_mutex); error = PTR_ERR(device); @@ -2190,7 +2190,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, switch (cmd) { case BTRFS_IOC_SCAN_DEV: mutex_lock(&uuid_mutex); - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); ret = PTR_ERR_OR_ZERO(device); mutex_unlock(&uuid_mutex); break; @@ -2204,7 +2204,7 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd, break; case BTRFS_IOC_DEVICES_READY: mutex_lock(&uuid_mutex); - device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ); + device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false); if (IS_ERR(device)) { mutex_unlock(&uuid_mutex); ret = PTR_ERR(device); diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 73753dae111a..433a490f2de8 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -681,12 +681,14 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices, return -EINVAL; } -static u8 *btrfs_sb_metadata_uuid_or_null(struct btrfs_super_block *sb) +static u8 *btrfs_sb_metadata_uuid_single_dev(struct btrfs_super_block *sb, + bool has_metadata_uuid, + bool single_dev) { - bool has_metadata_uuid = (btrfs_super_incompat_flags(sb) & - BTRFS_FEATURE_INCOMPAT_METADATA_UUID); + if (has_metadata_uuid || single_dev) + return sb->metadata_uuid; - return has_metadata_uuid ? sb->metadata_uuid : NULL; + return NULL; } u8 *btrfs_sb_fsid_ptr(struct btrfs_super_block *sb) @@ -775,8 +777,36 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( return NULL; } + +static void prepare_virtual_fsid(struct btrfs_super_block *disk_super, + const char *path) +{ + struct btrfs_fs_devices *fs_devices; + u8 vfsid[BTRFS_FSID_SIZE]; + bool dup_fsid = true; + + while (dup_fsid) { + dup_fsid = false; + generate_random_uuid(vfsid); + + list_for_each_entry(fs_devices, &fs_uuids, fs_list) { + if (!memcmp(vfsid, fs_devices->fsid, BTRFS_FSID_SIZE) || + !memcmp(vfsid, fs_devices->metadata_uuid, + BTRFS_FSID_SIZE)) + dup_fsid = true; + } + } + + memcpy(disk_super->metadata_uuid, disk_super->fsid, BTRFS_FSID_SIZE); + memcpy(disk_super->fsid, vfsid, BTRFS_FSID_SIZE); + + pr_info("BTRFS: virtual fsid (%pU) set for SINGLE_DEV device %s (real fsid %pU)\n", + disk_super->fsid, path, disk_super->metadata_uuid); +} + /* - * Add new device to list of registered devices + * Add new device to list of registered devices, or in case of a SINGLE_DEV + * device, also creates a virtual fsid to cope with same-fsid cases. * * Returns: * device pointer which was just added or updated when successful @@ -784,7 +814,7 @@ static struct btrfs_fs_devices *find_fsid_reverted_metadata( */ static noinline struct btrfs_device *device_list_add(const char *path, struct btrfs_super_block *disk_super, - bool *new_device_added) + bool *new_device_added, bool single_dev) { struct btrfs_device *device; struct btrfs_fs_devices *fs_devices = NULL; @@ -805,23 +835,32 @@ static noinline struct btrfs_device *device_list_add(const char *path, return ERR_PTR(error); } - if (fsid_change_in_progress) { - if (!has_metadata_uuid) - fs_devices = find_fsid_inprogress(disk_super); - else - fs_devices = find_fsid_changed(disk_super); - } else if (has_metadata_uuid) { - fs_devices = find_fsid_with_metadata_uuid(disk_super); + if (single_dev) { + if (has_metadata_uuid || fsid_change_in_progress) { + btrfs_err(NULL, + "SINGLE_DEV devices don't support the metadata_uuid feature\n"); + return ERR_PTR(-EINVAL); + } + prepare_virtual_fsid(disk_super, path); } else { - fs_devices = find_fsid_reverted_metadata(disk_super); - if (!fs_devices) - fs_devices = find_fsid(disk_super->fsid, NULL); + if (fsid_change_in_progress) { + if (!has_metadata_uuid) + fs_devices = find_fsid_inprogress(disk_super); + else + fs_devices = find_fsid_changed(disk_super); + } else if (has_metadata_uuid) { + fs_devices = find_fsid_with_metadata_uuid(disk_super); + } else { + fs_devices = find_fsid_reverted_metadata(disk_super); + if (!fs_devices) + fs_devices = find_fsid(disk_super->fsid, NULL); + } } - if (!fs_devices) { fs_devices = alloc_fs_devices(disk_super->fsid, - btrfs_sb_metadata_uuid_or_null(disk_super)); + btrfs_sb_metadata_uuid_single_dev(disk_super, + has_metadata_uuid, single_dev)); if (IS_ERR(fs_devices)) return ERR_CAST(fs_devices); @@ -1365,13 +1404,15 @@ int btrfs_forget_devices(dev_t devt) * and we are not allowed to call set_blocksize during the scan. The superblock * is read via pagecache */ -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, + bool mounting) { struct btrfs_super_block *disk_super; bool new_device_added = false; struct btrfs_device *device = NULL; struct block_device *bdev; u64 bytenr, bytenr_orig; + bool single_dev; int ret; lockdep_assert_held(&uuid_mutex); @@ -1410,7 +1451,17 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags) goto error_bdev_put; } - device = device_list_add(path, disk_super, &new_device_added); + single_dev = btrfs_super_compat_ro_flags(disk_super) & + BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV; + + if (!mounting && single_dev) { + pr_info("BTRFS: skipped non-mount scan on SINGLE_DEV device %s\n", + path); + btrfs_release_disk_super(disk_super); + return ERR_PTR(-EINVAL); + } + + device = device_list_add(path, disk_super, &new_device_added, single_dev); if (!IS_ERR(device) && new_device_added) btrfs_free_stale_devices(device->devt, device); @@ -2406,6 +2457,12 @@ int btrfs_get_dev_args_from_path(struct btrfs_fs_info *fs_info, args->devid = btrfs_stack_device_id(&disk_super->dev_item); memcpy(args->uuid, disk_super->dev_item.uuid, BTRFS_UUID_SIZE); + + /* + * Note that SINGLE_DEV devices are not handled in a special way here; + * device removal/replace is instead forbidden when such feature is + * present, this note is for future users/readers of this function. + */ if (btrfs_fs_incompat(fs_info, METADATA_UUID)) memcpy(args->fsid, disk_super->metadata_uuid, BTRFS_FSID_SIZE); else diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h index 0f87057bb575..b9856c801567 100644 --- a/fs/btrfs/volumes.h +++ b/fs/btrfs/volumes.h @@ -611,7 +611,8 @@ struct btrfs_block_group *btrfs_create_chunk(struct btrfs_trans_handle *trans, void btrfs_mapping_tree_free(struct extent_map_tree *tree); int btrfs_open_devices(struct btrfs_fs_devices *fs_devices, blk_mode_t flags, void *holder); -struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags); +struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, + bool mounting); int btrfs_forget_devices(dev_t devt); void btrfs_close_devices(struct btrfs_fs_devices *fs_devices); void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices); diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h index dbb8b96da50d..cb7a7cfe1ea9 100644 --- a/include/uapi/linux/btrfs.h +++ b/include/uapi/linux/btrfs.h @@ -313,6 +313,13 @@ struct btrfs_ioctl_fs_info_args { */ #define BTRFS_FEATURE_COMPAT_RO_BLOCK_GROUP_TREE (1ULL << 3) +/* + * Single devices (as flagged by the corresponding compat_ro flag) only + * gets scanned during mount time; also, a random fsid is generated for + * them, in order to cope with same-fsid filesystem mounts. + */ +#define BTRFS_FEATURE_COMPAT_RO_SINGLE_DEV (1ULL << 4) + #define BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF (1ULL << 0) #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL (1ULL << 1) #define BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS (1ULL << 2)
Btrfs doesn't currently support to mount 2 different devices holding the same filesystem - the fsid is used as a unique identifier in the driver. This case is supported though in some other common filesystems, like ext4; one of the reasons for which is not trivial supporting this case on btrfs is due to its multi-device filesystem nature, native RAID, etc. Supporting the same-fsid mounts has the advantage of allowing btrfs to be used in A/B partitioned devices, like mobile phones or the Steam Deck for example. Without this support, it's not safe for users to keep the same "image version" in both A and B partitions, a setup that is quite common for development, for example. Also, as a big bonus, it allows fs integrity check based on block devices for RO devices (whereas currently it is required that both have different fsid, breaking the block device hash comparison). Such same-fsid mounting is hereby added through the usage of the filesystem feature "single-dev" - when such feature is used, btrfs generates a random fsid for the filesystem and leverages the long-term present metadata_uuid infrastructure to enable the usage of this secondary virtual fsid, effectively requiring few non-invasive changes to the code and no new potential corner cases. In order to prevent more code complexity and corner cases, given the nature of this mechanism (single-devices), the single-dev feature is not allowed when the metadata_uuid flag is already present on the fs, or if the device is on fsid-change state. Device removal/replace is also disabled for devices presenting the single-dev feature. Suggested-by: John Schoenick <johns@valvesoftware.com> Suggested-by: Qu Wenruo <wqu@suse.com> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com> --- fs/btrfs/disk-io.c | 19 +++++++- fs/btrfs/fs.h | 3 +- fs/btrfs/ioctl.c | 18 +++++++ fs/btrfs/super.c | 8 ++-- fs/btrfs/volumes.c | 97 ++++++++++++++++++++++++++++++-------- fs/btrfs/volumes.h | 3 +- include/uapi/linux/btrfs.h | 7 +++ 7 files changed, 127 insertions(+), 28 deletions(-)