[PATCHSET,v24.0,00/21] xfs: atomic file updates

Hi all, This series creates a new FIEXCHANGE_RANGE system call to exchange ranges of bytes between two files atomically. This new functionality enables data storage programs to stage and commit file updates such that reader programs will see either the old contents or the new contents in their entirety, with no chance of torn writes. A successful call completion guarantees that the new contents will be seen even if the system fails. The ability to swap extent mappings between files in this manner is critical to supporting online filesystem repair, which is built upon the strategy of constructing a clean copy of a damaged structure and committing the new structure into the metadata file atomically. User programs will be able to update files atomically by opening an O_TMPFILE, reflinking the source file to it, making whatever updates they want to make, and exchange the relevant ranges of the temp file with the original file. If the updates are aligned with the file block size, a new (since v2) flag provides for exchanging only the written areas. Callers can arrange for the update to be rejected if the original file has been changed. The intent behind this new userspace functionality is to enable atomic rewrites of arbitrary parts of individual files. For years, application programmers wanting to ensure the atomicity of a file update had to write the changes to a new file in the same directory, fsync the new file, rename the new file on top of the old filename, and then fsync the directory. People get it wrong all the time, and $fs hacks abound. Here is the proposed manual page: IOCTL-FIEXCHANGE_RANGE(Linux Programmer's ManIOCTL-FIEXCHANGE_RANGE(2) NAME ioctl_fiexchange_range - exchange the contents of parts of two files SYNOPSIS #include <sys/ioctl.h> #include <linux/fiexchange.h> int ioctl(int file2_fd, FIEXCHANGE_RANGE, struct file_xchg_range *arg); DESCRIPTION Given a range of bytes in a first file file1_fd and a second range of bytes in a second file file2_fd, this ioctl(2) ex‐ changes the contents of the two ranges. Exchanges are atomic with regards to concurrent file opera‐ tions, so no userspace-level locks need to be taken to obtain consistent results. Implementations must guarantee that read‐ ers see either the old contents or the new contents in their entirety, even if the system fails. The exchange parameters are conveyed in a structure of the fol‐ lowing form: struct file_xchg_range { __s64 file1_fd; __s64 file1_offset; __s64 file2_offset; __s64 length; __u64 flags; __s64 file2_ino; __s64 file2_mtime; __s64 file2_ctime; __s32 file2_mtime_nsec; __s32 file2_ctime_nsec; __u64 pad[6]; }; The field pad must be zero. The fields file1_fd, file1_offset, and length define the first range of bytes to be exchanged. The fields file2_fd, file2_offset, and length define the second range of bytes to be exchanged. Both files must be from the same filesystem mount. If the two file descriptors represent the same file, the byte ranges must not overlap. Most disk-based filesystems require that the starts of both ranges must be aligned to the file block size. If this is the case, the ends of the ranges must also be so aligned unless the FILE_XCHG_RANGE_TO_EOF flag is set. The field flags control the behavior of the exchange operation. FILE_XCHG_RANGE_FILE2_FRESH Check the freshness of file2_fd after locking the file but before exchanging the contents. The sup‐ plied file2_ino field must match file2's inode num‐ ber, and the supplied file2_mtime, file2_mtime_nsec, file2_ctime, and file2_ctime_nsec fields must match the modification time and change time of file2. If they do not match, EBUSY will be returned. FILE_XCHG_RANGE_TO_EOF Ignore the length parameter. All bytes in file1_fd from file1_offset to EOF are moved to file2_fd, and file2's size is set to (file2_offset+(file1_length- file1_offset)). Meanwhile, all bytes in file2 from file2_offset to EOF are moved to file1 and file1's size is set to (file1_offset+(file2_length- file2_offset)). This option is not compatible with FILE_XCHG_RANGE_FULL_FILES. FILE_XCHG_RANGE_FSYNC Ensure that all modified in-core data in both file ranges and all metadata updates pertaining to the exchange operation are flushed to persistent storage before the call returns. Opening either file de‐ scriptor with O_SYNC or O_DSYNC will have the same effect. FILE_XCHG_RANGE_SKIP_FILE1_HOLES Skip sub-ranges of file1_fd that are known not to contain data. This facility can be used to imple‐ ment atomic scatter-gather writes of any complexity for software-defined storage targets. FILE_XCHG_RANGE_DRY_RUN Check the parameters and the feasibility of the op‐ eration, but do not change anything. FILE_XCHG_RANGE_COMMIT This flag is a combination of FILE_XCHG_RANGE_FILE2_FRESH | FILE_XCHG_RANGE_FSYNC and can be used to commit changes to file2_fd to persistent storage if and only if file2 has not changed. FILE_XCHG_RANGE_FULL_FILES Require that file1_offset and file2_offset are zero, and that the length field matches the lengths of both files. If not, EDOM will be returned. This option is not compatible with FILE_XCHG_RANGE_TO_EOF. FILE_XCHG_RANGE_NONATOMIC This flag relaxes the requirement that readers see only the old contents or the new contents in their entirety. If the system fails before all modified in-core data and metadata updates are persisted to disk, the contents of both file ranges after recov‐ ery are not defined and may be a mix of both. Do not use this flag unless the contents of both ranges are known to be identical and there are no other writers. RETURN VALUE On error, -1 is returned, and errno is set to indicate the er‐ ror. ERRORS Error codes can be one of, but are not limited to, the follow‐ ing: EBADF file1_fd is not open for reading and writing or is open for append-only writes; or file2_fd is not open for reading and writing or is open for append-only writes. EBUSY The inode number and timestamps supplied do not match file2_fd and FILE_XCHG_RANGE_FILE2_FRESH was set in flags. EDOM The ranges do not cover the entirety of both files, and FILE_XCHG_RANGE_FULL_FILES was set in flags. EINVAL The parameters are not correct for these files. This error can also appear if either file descriptor repre‐ sents a device, FIFO, or socket. Disk filesystems gen‐ erally require the offset and length arguments to be aligned to the fundamental block sizes of both files. EIO An I/O error occurred. EISDIR One of the files is a directory. ENOMEM The kernel was unable to allocate sufficient memory to perform the operation. ENOSPC There is not enough free space in the filesystem ex‐ change the contents safely. EOPNOTSUPP The filesystem does not support exchanging bytes between the two files. EPERM file1_fd or file2_fd are immutable. ETXTBSY One of the files is a swap file. EUCLEAN The filesystem is corrupt. EXDEV file1_fd and file2_fd are not on the same mounted filesystem. CONFORMING TO This API is Linux-specific. USE CASES Three use cases are imagined for this system call. The first is a filesystem defragmenter, which copies the con‐ tents of a file into another file and wishes to exchange the space mappings of the two files, provided that the original file has not changed. The flags NONATOMIC and FILE2_FRESH are recommended for this application. The second is a data storage program that wants to commit non- contiguous updates to a file atomically. This can be done by creating a temporary file, calling FICLONE(2) to share the con‐ tents, and staging the updates into the temporary file. Either of the FULL_FILES or TO_EOF flags are recommended, along with FSYNC. Depending on the application's locking design, the flags FILE2_FRESH or COMMIT may be applicable here. The tempo‐ rary file can be deleted or punched out afterwards. The third is a software-defined storage host (e.g. a disk juke‐ box) which implements an atomic scatter-gather write command. Provided the exported disk's logical block size matches the file's allocation unit size, this can be done by creating a temporary file and writing the data at the appropriate offsets. Use this call with the SKIP_HOLES flag to exchange only the blocks involved in the write command. The use of the FSYNC flag is recommended here. The temporary file should be deleted or punched out completely before being reused to stage another write. NOTES Some filesystems may limit the amount of data or the number of extents that can be exchanged in a single call. SEE ALSO ioctl(2) Linux 2022-12-31 IOCTL-FIEXCHANGE_RANGE(2) The reference implementation in XFS creates a new log incompat feature and log intent items to track high level progress of swapping ranges of two files and finish interrupted work if the system goes down. Sample code can be found in the corresponding changes to xfs_io to exercise the use case mentioned above. Note that this function is /not/ the O_DIRECT atomic file writes concept that has also been floating around for years. This RFC is constructed entirely in software, which means that there are no limitations other than the general filesystem limits. As a side note, the original motivation behind the kernel functionality is online repair of file-based metadata. The atomic file swap is implemented as an atomic inode fork swap, which means that we can implement online reconstruction of extended attributes and directories by building a new one in another inode and atomically swap the contents. Subsequent patchsets adapt the online filesystem repair code to use atomic extent swapping. This enables repair functions to construct a clean copy of a directory, xattr information, symbolic links, realtime bitmaps, and realtime summary information in a temporary inode. If this completes successfully, the new contents can be swapped atomically into the inode being repaired. This is essential to avoid making corruption problems worse if the system goes down in the middle of running repair. This patchset also ports the old XFS extent swap ioctl interface to use the new extent swap code. For userspace, this series also includes the userspace pieces needed to test the new functionality, and a sample implementation of atomic file updates. Question: Should we really bother with fsdevel bikeshedding? Most filesystems cannot support this functionality, so we could keep it private to XFS for now. If you're going to start using this mess, you probably ought to just pull from my git trees, which are linked below. This is an extraordinary way to destroy everything. Enjoy! Comments and questions are, as always, welcome. --D kernel git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=atomic-file-updates xfsprogs git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=atomic-file-updates fstests git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfstests-dev.git/log/?h=atomic-file-updates xfsdocs git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-documentation.git/log/?h=atomic-file-updates --- Documentation/filesystems/vfs.rst | 16 fs/ioctl.c | 27 + fs/remap_range.c | 296 ++++++++ fs/xfs/Makefile | 3 fs/xfs/libxfs/xfs_bmap.h | 4 fs/xfs/libxfs/xfs_defer.c | 7 fs/xfs/libxfs/xfs_defer.h | 3 fs/xfs/libxfs/xfs_errortag.h | 4 fs/xfs/libxfs/xfs_format.h | 15 fs/xfs/libxfs/xfs_fs.h | 2 fs/xfs/libxfs/xfs_log_format.h | 80 ++ fs/xfs/libxfs/xfs_log_recover.h | 2 fs/xfs/libxfs/xfs_sb.c | 3 fs/xfs/libxfs/xfs_swapext.c | 1258 ++++++++++++++++++++++++++++++++++++ fs/xfs/libxfs/xfs_swapext.h | 170 +++++ fs/xfs/libxfs/xfs_symlink_remote.c | 47 + fs/xfs/libxfs/xfs_symlink_remote.h | 1 fs/xfs/libxfs/xfs_trans_space.h | 4 fs/xfs/xfs_bmap_util.c | 620 ------------------ fs/xfs/xfs_bmap_util.h | 3 fs/xfs/xfs_error.c | 3 fs/xfs/xfs_file.c | 94 ++- fs/xfs/xfs_inode.c | 13 fs/xfs/xfs_inode.h | 6 fs/xfs/xfs_ioctl.c | 102 +-- fs/xfs/xfs_ioctl.h | 4 fs/xfs/xfs_ioctl32.c | 11 fs/xfs/xfs_linux.h | 5 fs/xfs/xfs_log.c | 47 + fs/xfs/xfs_log.h | 10 fs/xfs/xfs_log_priv.h | 3 fs/xfs/xfs_log_recover.c | 5 fs/xfs/xfs_mount.c | 11 fs/xfs/xfs_mount.h | 7 fs/xfs/xfs_rtalloc.c | 136 ++++ fs/xfs/xfs_rtalloc.h | 3 fs/xfs/xfs_super.c | 19 + fs/xfs/xfs_swapext_item.c | 657 +++++++++++++++++++ fs/xfs/xfs_swapext_item.h | 56 ++ fs/xfs/xfs_symlink.c | 49 - fs/xfs/xfs_trace.c | 2 fs/xfs/xfs_trace.h | 351 ++++++++++ fs/xfs/xfs_xattr.c | 6 fs/xfs/xfs_xchgrange.c | 964 ++++++++++++++++++++++++++++ fs/xfs/xfs_xchgrange.h | 40 + include/linux/fs.h | 14 include/uapi/linux/fiexchange.h | 101 +++ 47 files changed, 4473 insertions(+), 811 deletions(-) create mode 100644 fs/xfs/libxfs/xfs_swapext.c create mode 100644 fs/xfs/libxfs/xfs_swapext.h create mode 100644 fs/xfs/xfs_swapext_item.c create mode 100644 fs/xfs/xfs_swapext_item.h create mode 100644 fs/xfs/xfs_xchgrange.c create mode 100644 fs/xfs/xfs_xchgrange.h create mode 100644 include/uapi/linux/fiexchange.h

Message ID	167243843494.699466.5163281976943635014.stgit@magnolia (mailing list archive)
Headers	show Return-Path: <linux-fsdevel-owner@vger.kernel.org> Subject: [PATCHSET v24.0 00/21] xfs: atomic file updates From: "Darrick J. Wong" <djwong@kernel.org> To: djwong@kernel.org Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org Date: Fri, 30 Dec 2022 14:13:55 -0800 Message-ID: <167243843494.699466.5163281976943635014.stgit@magnolia> In-Reply-To: <Y69Unb7KRM5awJoV@magnolia> References: <Y69Unb7KRM5awJoV@magnolia> User-Agent: StGit/0.19 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Precedence: bulk
Series	xfs: atomic file updates \| expand [PATCHSET,v24.0,00/21] xfs: atomic file updates [02/21] xfs: create a new helper to return a file's allocation unit [03/21] xfs: refactor non-power-of-two alignment checks [04/21] xfs: parameterize all the incompat log feature helpers [05/21] xfs: create a log incompat flag for atomic extent swapping [01/21] vfs: introduce new file range exchange ioctl [11/21] xfs: port xfs_swap_extents_rmap to our new code [10/21] xfs: add error injection to test swapext recovery [09/21] xfs: add a ->xchg_file_range handler [08/21] xfs: enable xlog users to toggle atomic extent swapping [07/21] xfs: create deferred log items for extent swapping [06/21] xfs: introduce a swap-extent log intent item [12/21] xfs: consolidate all of the xfs_swap_extent_forks code [13/21] xfs: port xfs_swap_extent_forks to use xfs_swapext_req [14/21] xfs: allow xfs_swap_range to use older extent swap algorithms [15/21] xfs: remove old swap extents implementation [16/21] xfs: condense extended attributes after an atomic swap [17/21] xfs: condense directories after an atomic swap [18/21] xfs: condense symbolic links after an atomic swap [19/21] xfs: make atomic extent swapping support realtime files [20/21] xfs: support non-power-of-two rtextsize with exchange-range [21/21] xfs: enable atomic swapext feature

[PATCHSET,v24.0,00/21] xfs: atomic file updates

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