From: Dominique Martinet dominique.martinet@atmark-techno.com
btrfs_read_extent_reg correctly computed the extent offset in the BTRFS_COMPRESS_NONE case, but did not account for the 'offset - key.offset' part correctly in the compressed case, making the function read incorrect data.
In the case I examined, the last 4k of a file was corrupted and contained data from a few blocks prior, e.g. reading a 10k file with a single extent: btrfs_file_read() -> btrfs_read_extent_reg (aligned part loop, until 8k) -> read_and_truncate_page -> btrfs_read_extent_reg (re-reads the last extent from 8k to the end, incorrectly reading the first 2k of data)
This can be reproduced as follow: $ truncate -s 200M btr $ mount btr -o compress /mnt $ pat() { dd if=/dev/zero bs=1M count=$1 iflag=count_bytes status=none | tr '\0' "\$2"; } $ { pat 4K 1; pat 4K 2; pat 2K 3; } > /mnt/file $ sync $ filefrag -v /mnt/file File size of /mnt/file is 10240 (3 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 2: 3328.. 3330: 3: last,encoded,eof $ umount /mnt
Then in u-boot: => load scsi 0 2000000 file 10240 bytes read in 3 ms (3.3 MiB/s) => md 2001ff0 02001ff0: 02020202 02020202 02020202 02020202 ................ 02002000: 01010101 01010101 01010101 01010101 ................ 02002010: 01010101 01010101 01010101 01010101 ................
(02002000 onwards should contain '03' pattern but went back to 01, start of the extent)
After patch, data is read properly: => md 2001ff0 02001ff0: 02020202 02020202 02020202 02020202 ................ 02002000: 03030303 03030303 03030303 03030303 ................ 02002010: 03030303 03030303 03030303 03030303 ................
Note that the code previously (before commit e3427184f38a ("fs: btrfs: Implement btrfs_file_read()")) did not split that read in two, so this is a regression even if the previous code might not have been handling offsets correctly either (something that booted now fails to boot)
Fixes: a26a6bedafcf ("fs: btrfs: Introduce btrfs_read_extent_inline() and btrfs_read_extent_reg()") Signed-off-by: Dominique Martinet dominique.martinet@atmark-techno.com --- Changes in v2: - Keep offset decomposition explicit where it is used - Add reproducer/clarify explanation in commit message - Drop other patches temporarily - Link to v1: https://lore.kernel.org/r/20230418-btrfs-extent-reads-v1-0-47ba9839f0cc@code... --- fs/btrfs/inode.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 40025662f250..38e285bf94b0 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -511,7 +511,9 @@ int btrfs_read_extent_reg(struct btrfs_path *path, if (ret < dsize) memset(dbuf + ret, 0, dsize - ret); /* Then copy the needed part */ - memcpy(dest, dbuf + btrfs_file_extent_offset(leaf, fi), len); + memcpy(dest, + dbuf + btrfs_file_extent_offset(leaf, fi) + offset - key.offset, + len); ret = len; out: free(cbuf);
--- base-commit: 5db4972a5bbdbf9e3af48ffc9bc4fec73b7b6a79 change-id: 20230418-btrfs-extent-reads-e2df6e329ad4
Best regards,