| Message ID | 20170209124433.2626-9-jack@suse.cz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hello, Jan. On Thu, Feb 09, 2017 at 01:44:31PM +0100, Jan Kara wrote: > When block device is closed, we call inode_detach_wb() in __blkdev_put() > which sets inode->i_wb to NULL. That is contrary to expectations that > inode->i_wb stays valid once set during the whole inode's lifetime and > leads to oops in wb_get() in locked_inode_to_wb_and_lock_list() because > inode_to_wb() returned NULL. > > The reason why we called inode_detach_wb() is not valid anymore though. > BDI is guaranteed to stay along until we call bdi_put() from > bdev_evict_inode() so we can postpone calling inode_detach_wb() to that > moment. A complication is that i_wb can point to non-root wb_writeback > structure and in that case we do need to clean it up as bdi_unregister() > blocks waiting for all non-root wb_writeback references to get dropped. > Thus this i_wb reference could block device removal e.g. from > __scsi_remove_device() (which indirectly ends up calling > bdi_unregister()). We cannot rely on block device inode to go away soon > (and thus i_wb reference to get dropped) as the device may got > hot-removed e.g. under a mounted filesystem. We deal with these issues > by switching block device inode from non-root wb_writeback structure to > bdi->wb when needed. Since this is rather expensive (requires > synchronize_rcu()) we do the switching only in del_gendisk() when we > know the device is going away. So, the only reason cgwb_bdi_destroy() is synchronous is because bdi destruction was synchronous. Now that bdi is properly reference counted and can be decoupled from gendisk / q destruction, I can't think of a reason to keep cgwb destruction synchronous. Switching wb's on destruction is kinda clumsy and it almost always hurts to expose synchronize_rcu() in userland visible paths. Wouldn't something like the following work? * Remove bdi->usage_cnt and the synchronous waiting in cgwb_bdi_destroy(). * Instead, make cgwb's hold bdi->refcnt and put it from cgwb_release_workfn(). Then, we don't have to switch during shutdown and can just let things drain. Thanks.
On Sun 12-02-17 13:40:27, Tejun Heo wrote: > Hello, Jan. > > On Thu, Feb 09, 2017 at 01:44:31PM +0100, Jan Kara wrote: > > When block device is closed, we call inode_detach_wb() in __blkdev_put() > > which sets inode->i_wb to NULL. That is contrary to expectations that > > inode->i_wb stays valid once set during the whole inode's lifetime and > > leads to oops in wb_get() in locked_inode_to_wb_and_lock_list() because > > inode_to_wb() returned NULL. > > > > The reason why we called inode_detach_wb() is not valid anymore though. > > BDI is guaranteed to stay along until we call bdi_put() from > > bdev_evict_inode() so we can postpone calling inode_detach_wb() to that > > moment. A complication is that i_wb can point to non-root wb_writeback > > structure and in that case we do need to clean it up as bdi_unregister() > > blocks waiting for all non-root wb_writeback references to get dropped. > > Thus this i_wb reference could block device removal e.g. from > > __scsi_remove_device() (which indirectly ends up calling > > bdi_unregister()). We cannot rely on block device inode to go away soon > > (and thus i_wb reference to get dropped) as the device may got > > hot-removed e.g. under a mounted filesystem. We deal with these issues > > by switching block device inode from non-root wb_writeback structure to > > bdi->wb when needed. Since this is rather expensive (requires > > synchronize_rcu()) we do the switching only in del_gendisk() when we > > know the device is going away. > > So, the only reason cgwb_bdi_destroy() is synchronous is because bdi > destruction was synchronous. Now that bdi is properly reference > counted and can be decoupled from gendisk / q destruction, I can't > think of a reason to keep cgwb destruction synchronous. Switching > wb's on destruction is kinda clumsy and it almost always hurts to > expose synchronize_rcu() in userland visible paths. > > Wouldn't something like the following work? > > * Remove bdi->usage_cnt and the synchronous waiting in > cgwb_bdi_destroy(). > > * Instead, make cgwb's hold bdi->refcnt and put it from > cgwb_release_workfn(). > > Then, we don't have to switch during shutdown and can just let things > drain. At first sight this looks workable and would mean less special code so I like it. I'll experiment with it and see how it works out. Honza
diff --git a/block/genhd.c b/block/genhd.c index 68c613edb93a..721921a140cc 100644 --- a/block/genhd.c +++ b/block/genhd.c @@ -649,13 +649,13 @@ void del_gendisk(struct gendisk *disk) DISK_PITER_INCL_EMPTY | DISK_PITER_REVERSE); while ((part = disk_part_iter_next(&piter))) { invalidate_partition(disk, part->partno); - bdev_unhash_inode(part_devt(part)); + bdev_cleanup_inode(part_devt(part)); delete_partition(disk, part->partno); } disk_part_iter_exit(&piter); invalidate_partition(disk, 0); - bdev_unhash_inode(disk_devt(disk)); + bdev_cleanup_inode(disk_devt(disk)); set_capacity(disk, 0); disk->flags &= ~GENHD_FL_UP; diff --git a/fs/block_dev.c b/fs/block_dev.c index 360439373a66..65ac3a60ac8e 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -884,6 +884,8 @@ static void bdev_evict_inode(struct inode *inode) spin_lock(&bdev_lock); list_del_init(&bdev->bd_list); spin_unlock(&bdev_lock); + /* Detach inode from wb early as bdi_put() may free bdi->wb */ + inode_detach_wb(inode); if (bdev->bd_bdi != &noop_backing_dev_info) bdi_put(bdev->bd_bdi); } @@ -960,13 +962,14 @@ static LIST_HEAD(all_bdevs); * If there is a bdev inode for this device, unhash it so that it gets evicted * as soon as last inode reference is dropped. */ -void bdev_unhash_inode(dev_t dev) +void bdev_cleanup_inode(dev_t dev) { struct inode *inode; inode = ilookup5(blockdev_superblock, hash(dev), bdev_test, &dev); if (inode) { remove_inode_hash(inode); + inode_switch_to_default_wb_sync(inode); iput(inode); } } @@ -1874,12 +1877,6 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) kill_bdev(bdev); bdev_write_inode(bdev); - /* - * Detaching bdev inode from its wb in __destroy_inode() - * is too late: the queue which embeds its bdi (along with - * root wb) can be gone as soon as we put_disk() below. - */ - inode_detach_wb(bdev->bd_inode); } if (bdev->bd_contains == bdev) { if (disk->fops->release) diff --git a/include/linux/fs.h b/include/linux/fs.h index 319fb76f9081..f8c86b9c31d5 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2344,7 +2344,7 @@ extern struct kmem_cache *names_cachep; #ifdef CONFIG_BLOCK extern int register_blkdev(unsigned int, const char *); extern void unregister_blkdev(unsigned int, const char *); -extern void bdev_unhash_inode(dev_t dev); +extern void bdev_cleanup_inode(dev_t dev); extern struct block_device *bdget(dev_t); extern struct block_device *bdgrab(struct block_device *bdev); extern void bd_set_size(struct block_device *, loff_t size); diff --git a/include/linux/writeback.h b/include/linux/writeback.h index 0d3ba83a0f7f..6d27b78c9a79 100644 --- a/include/linux/writeback.h +++ b/include/linux/writeback.h @@ -237,6 +237,7 @@ static inline void inode_attach_wb(struct inode *inode, struct page *page) static inline void inode_detach_wb(struct inode *inode) { if (inode->i_wb) { + WARN_ON_ONCE(!(inode->i_state & I_CLEAR)); wb_put(inode->i_wb); inode->i_wb = NULL; }
When block device is closed, we call inode_detach_wb() in __blkdev_put() which sets inode->i_wb to NULL. That is contrary to expectations that inode->i_wb stays valid once set during the whole inode's lifetime and leads to oops in wb_get() in locked_inode_to_wb_and_lock_list() because inode_to_wb() returned NULL. The reason why we called inode_detach_wb() is not valid anymore though. BDI is guaranteed to stay along until we call bdi_put() from bdev_evict_inode() so we can postpone calling inode_detach_wb() to that moment. A complication is that i_wb can point to non-root wb_writeback structure and in that case we do need to clean it up as bdi_unregister() blocks waiting for all non-root wb_writeback references to get dropped. Thus this i_wb reference could block device removal e.g. from __scsi_remove_device() (which indirectly ends up calling bdi_unregister()). We cannot rely on block device inode to go away soon (and thus i_wb reference to get dropped) as the device may got hot-removed e.g. under a mounted filesystem. We deal with these issues by switching block device inode from non-root wb_writeback structure to bdi->wb when needed. Since this is rather expensive (requires synchronize_rcu()) we do the switching only in del_gendisk() when we know the device is going away. Also add a warning to catch if someone uses inode_detach_wb() in a dangerous way. Reported-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com> Signed-off-by: Jan Kara <jack@suse.cz> --- block/genhd.c | 4 ++-- fs/block_dev.c | 11 ++++------- include/linux/fs.h | 2 +- include/linux/writeback.h | 1 + 4 files changed, 8 insertions(+), 10 deletions(-)