Hi all, This series creates a new XFS_IOC_EXCHANGE_RANGE ioctl 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 exchange file fork 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. The ioctls exist to facilitate testing of the new functionality and to enable future application program designs. 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. Note that application software must quiesce writes to the file while it stages an atomic update. This will be addressed by a subsequent series. This mechanism solves the clunkiness of two existing atomic file update mechanisms: for O_TRUNC + rewrite, this eliminates the brief period where other programs can see an empty file. For create tempfile + rename, the need to copy file attributes and extended attributes for each file update is eliminated. However, this method introduces its own awkwardness -- any program initiating an exchange now needs to have a way to signal to other programs that the file contents have changed. For file access mediated via read and write, fanotify or inotify are probably sufficient. For mmaped files, that may not be fast enough. 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 untorn file writes concept that has also been floating around for years. It is also not the RWF_ATOMIC patchset that has been shared. 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 content exchange is implemented as an atomic exchange of file fork mappings, which means that we can implement online reconstruction of extended attributes and directories by building a new one in another inode and exchanging the contents. Subsequent patchsets adapt the online filesystem repair code to use atomic file exchanges. 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 committed 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. For userspace, this series also includes the userspace pieces needed to test the new functionality, and a sample implementation of atomic file updates. If you're going to start using this code, I strongly recommend pulling from my git trees, which are linked below. This has been running on the djcloud for months with no problems. 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 --- Commits in this patchset: * misc: split swapext and exchangerange * misc: change xfs_io -c swapext to exchangerange * generic/709,710: rework these for exchangerange vs. quota testing * generic/711,xfs/537: actually fork these tests for exchange-range * generic/717: remove obsolete check * ltp/{fsstress,fsx}: make the exchangerange naming consistent * misc: flip HAVE_XFS_IOC_EXCHANGE_RANGE logic * src/fiexchange.h: update XFS_IOC_EXCHANGE_RANGE definitions * xfs/122: fix for exchrange conversion * xfs/206: screen out exchange-range from golden output * exchangerange: make sure that we don't swap unwritten extents unless they're part of a rt extent --- common/rc | 2 common/xfs | 2 configure.ac | 2 include/builddefs.in | 2 ltp/Makefile | 4 - ltp/fsstress.c | 34 +++---- ltp/fsx.c | 43 +++------ m4/package_xfslibs.m4 | 15 ++- src/Makefile | 4 - src/fiexchange.h | 84 ++++------------- src/global.h | 12 ++ src/vfs/Makefile | 4 - src/xfsfind.c | 1 tests/generic/1221 | 45 +++++++++ tests/generic/1221.out | 2 tests/generic/709 | 12 +- tests/generic/710 | 14 +-- tests/generic/710.out | 2 tests/generic/711 | 4 - tests/generic/712 | 10 +- tests/generic/713 | 42 ++++----- tests/generic/713.out | 38 ++++---- tests/generic/714 | 40 ++++---- tests/generic/714.out | 34 ++++--- tests/generic/715 | 26 +++-- tests/generic/715.out | 14 +-- tests/generic/716 | 4 - tests/generic/717 | 39 ++++---- tests/generic/717.out | 32 +++---- tests/generic/718 | 12 +- tests/generic/718.out | 2 tests/generic/719 | 4 - tests/generic/720 | 10 +- tests/generic/721 | 2 tests/generic/722 | 8 +- tests/generic/723 | 12 +- tests/generic/724 | 10 +- tests/generic/725 | 4 - tests/generic/726 | 4 - tests/generic/727 | 4 - tests/xfs/1213 | 73 +++++++++++++++ tests/xfs/1213.out | 2 tests/xfs/1214 | 232 ++++++++++++++++++++++++++++++++++++++++++++++++ tests/xfs/1214.out | 2 tests/xfs/1215 | 89 ++++++++++++++++++ tests/xfs/1215.out | 13 +++ tests/xfs/122.out | 6 + tests/xfs/206 | 1 tests/xfs/300 | 2 tests/xfs/443 | 4 - tests/xfs/789 | 4 - tests/xfs/790 | 10 +- tests/xfs/790.out | 2 tests/xfs/791 | 10 +- tests/xfs/791.out | 2 tests/xfs/792 | 4 - tests/xfs/795 | 2 57 files changed, 748 insertions(+), 349 deletions(-) create mode 100755 tests/generic/1221 create mode 100644 tests/generic/1221.out create mode 100755 tests/xfs/1213 create mode 100644 tests/xfs/1213.out create mode 100755 tests/xfs/1214 create mode 100644 tests/xfs/1214.out create mode 100755 tests/xfs/1215 create mode 100644 tests/xfs/1215.out
