tl;dr: The proposed S_IOMAP_IMMUTABLE mechanism The daxfile proposal a few weeks back [1] sought to piggy back on the swapfile implementation to approximate a block map immutable file. This is an idea Dave originated last year to solve the dax "flush from userspace" problem [2]. The discussion yielded several results. First, Christoph pointed out that swapfiles are subtly broken [3]. Second, Darrick [4] and Dave [5] proposed how to properly implement a block map immutable file. Finally, Dave identified some improvements to swapfiles that can be built on the block-map-immutable mecahanism. These patches seek to implement the first part of the proposal and save the swapfile work to build on top once the base mechanism is complete. While the initial motivation for this feature is support for byte-addressable updates of persistent memory and managing cache maintenance from userspace, the applications of the feature are broader. In addition to being the start of a better swapfile mechanism it can also support a DMA-to-storage use case. This use case enables data-acquisition hardware to DMA directly to a storage device address while being safe in the knowledge that storage mappings will not change. These patches are relative to Darrick's 'devel' tree. Patch 3 is likely wrong in the way it sets the new XFS_DIFLAG2_IOMAP_IMMUTABLE flag, but seems to work with a basic test. The test just turns the flag on and off, checks that the file is fully allocated and immutable, and validates that the state persists over a umount / mount cycle. A proper xfstest is in the works, but comments on this first draft are welcome. [1]: https://lkml.org/lkml/2017/6/16/790 [2]: https://lkml.org/lkml/2016/9/11/159 [3]: https://lkml.org/lkml/2017/6/18/31 [4]: https://lkml.org/lkml/2017/6/20/49 [5]: https://www.spinics.net/lists/linux-xfs/msg07871.html --- Dan Williams (3): fs, xfs: introduce S_IOMAP_IMMUTABLE fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP xfs: persist S_IOMAP_IMMUTABLE in di_flags2 fs/attr.c | 10 ++++ fs/namei.c | 3 + fs/open.c | 28 ++++++++++++ fs/read_write.c | 3 + fs/xfs/libxfs/xfs_format.h | 5 ++ fs/xfs/xfs_bmap_util.c | 98 ++++++++++++++++++++++++++++++++++++++++++- fs/xfs/xfs_bmap_util.h | 4 +- fs/xfs/xfs_file.c | 14 ++++-- fs/xfs/xfs_ioctl.c | 10 ++++ fs/xfs/xfs_iops.c | 8 ++-- include/linux/falloc.h | 3 + include/linux/fs.h | 2 + include/uapi/linux/falloc.h | 19 ++++++++ mm/filemap.c | 9 ++++ 14 files changed, 200 insertions(+), 16 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
