From: Omar Sandoval <osandov@xxxxxx> Hello, This series adds an API for reading compressed data on a filesystem without decompressing it as well as support for writing compressed data directly to the filesystem. It is based on my previous series which added a Btrfs-specific ioctl [1], but it is now an extension to preadv2()/pwritev2() as suggested by Dave Chinner [2]. I've included a man page patch describing the API in detail. Test cases and examples programs are available [3]. The use case that I have in mind is Btrfs send/receive: currently, when sending data from one compressed filesystem to another, the sending side decompresses the data and the receiving side recompresses it before writing it out. This is wasteful and can be avoided if we can just send and write compressed extents. The send part will be implemented in a separate series, as this API can stand alone. Patches 1 and 2 add the VFS support. Patch 3 is a Btrfs prep patch. Patch 4 implements encoded reads for Btrfs, and patch 5 implements encoded writes. Changes from v1 [4]: - Encoded reads are now also implemented. - The encoded_iov structure now includes metadata for referring to a subset of decoded data. This is required to handle certain cases where a compressed extent is truncated, hole punched, or otherwise sliced up and Btrfs chooses to reflect this in metadata instead of decompressing the whole extent and rewriting the pieces. We call these "bookend extents" in Btrfs, but any filesystem supporting transparent encoding is likely to have a similar concept. - The behavior of the filesystem when the decompressed data is longer than or shorter than expected is more strictly defined (truncate and zero extend, respectively). - As pointed out by Jann Horn [5], the capability check done at read/write time in v1 was incorrect; v2 adds an explicit open flag (which can be changed with fcntl()). As this can be trivially combined with O_CLOEXEC, I did not add any sort of automatic clearing on exec. I wanted to get the ball rolling on reviewing the interface, so the Btrfs implementation has a couple of smaller todos: - Encoded reads do not yet implement repair for disk/checksum failures. - Encoded writes do not yet support inline extents or bookend extents. This is based on v5.4-rc3 Please share any comments on the API or implementation. Thanks! 1: https://lore.kernel.org/linux-fsdevel/cover.1567623877.git.osandov@xxxxxx/ 2: https://lore.kernel.org/linux-fsdevel/20190906212710.GI7452@vader/ 3: https://github.com/osandov/xfstests/tree/rwf-encoded 4: https://lore.kernel.org/linux-btrfs/cover.1568875700.git.osandov@xxxxxx/ 5: https://lore.kernel.org/linux-btrfs/CAG48ez2GKv15Uj6Wzv0sG5v2bXyrSaCtRTw5Ok_ovja_CiO_fQ@xxxxxxxxxxxxxx/ Omar Sandoval (5): fs: add O_ENCODED open flag fs: add RWF_ENCODED for reading/writing compressed data btrfs: generalize btrfs_lookup_bio_sums_dio() btrfs: implement RWF_ENCODED reads btrfs: implement RWF_ENCODED writes fs/btrfs/compression.c | 6 +- fs/btrfs/compression.h | 5 +- fs/btrfs/ctree.h | 9 +- fs/btrfs/file-item.c | 18 +- fs/btrfs/file.c | 52 ++- fs/btrfs/inode.c | 663 ++++++++++++++++++++++++++++++- fs/fcntl.c | 10 +- fs/namei.c | 4 + include/linux/fcntl.h | 2 +- include/linux/fs.h | 14 + include/uapi/asm-generic/fcntl.h | 4 + include/uapi/linux/fs.h | 26 +- mm/filemap.c | 82 +++- 13 files changed, 851 insertions(+), 44 deletions(-) -- 2.23.0
