Re: [PATCH] missing initialization in btrfs_check_shared

[Filipe Manana <fdmanana@gmail.com>]

On Wed, Mar 14, 2018 at 3:03 PM, Edmund Nadolski <enadolski@suse.com> wrote:
> This patch addresses an issue that causes fiemap to falsely
> report a shared extent.  The test case is as follows:
>
> # cat do_xfs_io
> xfs_io -f -d -c "pwrite -b 16k 0 64k" -c "fiemap -v" /media/scratch/file5
> sync
> xfs_io  -c "fiemap -v" /media/scratch/file5
>
> which gives the resulting output:
>
> # . do_xfs_io
> wrote 65536/65536 bytes at offset 0
> 64 KiB, 4 ops; 0.0000 sec (121.359 MiB/sec and 7766.9903 ops/sec)
> /media/scratch/file5:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..127]:        24576..24703       128 0x2001
> /media/scratch/file5:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..127]:        24576..24703       128   0x1
>
> This is because btrfs_check_shared calls find_parent_nodes
> repeatedly in a loop, passing a share_check struct to report
> the count of shared extent. But btrfs_check_shared does not
> re-initialize the count value to zero for subsequent calls
> from the loop, resulting in a false share count value. This
> is a regressive behavior from 4.13.
>
> With proper re-initialization the test result is as follows:
>
> # . do_xfs_io
> wrote 65536/65536 bytes at offset 0
> 64 KiB, 4 ops; 0.0000 sec (110.035 MiB/sec and 7042.2535 ops/sec)
> /media/scratch/file5:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..127]:        24576..24703       128   0x1
> /media/scratch/file5:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..127]:        24576..24703       128   0x1
>
> which corrects the regression.

All this information, about reproducer, which kernel/commit introduced
the regression, etc, should go into the changelog of the patch, not
the cover letter, since the cover letter won't go to the git
repository.

>
> Edmund Nadolski (1):
>   btrfs: add missing initialization in btrfs_check_shared
>
>  fs/btrfs/backref.c | 1 +
>  1 file changed, 1 insertion(+)
>
> --
> 2.10.2
>
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-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”