Re: [PATCH 3/4] xfs/420: fix occasional test failures due to pagecache readahead

[Zorro Lang <zlang@redhat.com>]

On Tue, Mar 15, 2022 at 08:30:20PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Every now and then, this test fails with this golden output:
> 
> --- xfs/420.out
> +++ xfs/420.out.bad
> @@ -29,7 +29,7 @@
>  Whence Result
>  DATA   0
>  HOLE   131072
> -DATA   196608
> +DATA   192512
>  HOLE   262144
>  Compare files
>  c2803804acc9936eef8aab42c119bfac  SCRATCH_MNT/test-420/file1

Looks like this part easy to cause `git am` misunderstanding[1], Hmm...
any method to deal with that?

[1]
Applying: xfs/420: fix occasional test failures due to pagecache readahead
error: 420.out: does not exist in index
Patch failed at 0001 xfs/420: fix occasional test failures due to pagecache readahead
hint: Use 'git am --show-current-patch=diff' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

> 
> Curiously, the file checksums always match, and it's not *forbidden* for
> the page cache to have a page backing an unwritten extent that hasn't
> been written.
> 
> The condition that this test cares about is that block 3 (192k-256k) are
> reported by SEEK_DATA as data even if the data fork has a hole and the
> COW fork has an unwritten extent.  Matthew Wilcox thinks this is a side
> effect of readahead.
> 
> To fix this occasional false failure, call SEEK_DATA and SEEK_HOLE only
> on the offsets that we care about.
> 
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  tests/xfs/420 |   33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> 
> diff --git a/tests/xfs/420 b/tests/xfs/420
> index 12b17588..d38772c9 100755
> --- a/tests/xfs/420
> +++ b/tests/xfs/420
> @@ -50,6 +50,24 @@ _scratch_mount >> $seqres.full 2>&1
>  testdir=$SCRATCH_MNT/test-$seq
>  mkdir $testdir
>  
> +# pagecache readahead can sometimes cause extra pages to be inserted into the
> +# file mapping where we have an unwritten extent in the COW fork.  Call lseek
> +# on each $blksz offset that interests us (as opposed to the whole file) so
> +# that these extra pages are not disclosed.
> +#
> +# The important thing we're testing is that SEEK_DATA reports block 3 as data
> +# when the COW fork has an unwritten mapping and the data fork has a hole.
> +exercise_lseek() {
> +	echo "Seek holes and data in file1"
> +	$XFS_IO_PROG -c "seek -d 0" $testdir/file1
> +	$XFS_IO_PROG -c "seek -h $((2 * blksz))" $testdir/file1 | sed -e '/Whence/d'
> +	echo "Seek holes and data in file2"
> +	$XFS_IO_PROG -c "seek -d 0" $testdir/file2
> +	$XFS_IO_PROG -c "seek -h $((2 * blksz))" $testdir/file2 | sed -e '/Whence/d'
> +	$XFS_IO_PROG -c "seek -d $((3 * blksz))" $testdir/file2 | sed -e '/Whence/d'
> +	$XFS_IO_PROG -c "seek -h $((4 * blksz))" $testdir/file2 | sed -e '/Whence/d'
> +}
> +
>  blksz=65536
>  nr=8
>  filesize=$((blksz * nr))
> @@ -83,10 +101,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
>  
> -echo "Seek holes and data in file1"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
> -echo "Seek holes and data in file2"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
> +exercise_lseek
>  
>  echo "Compare files"
>  md5sum $testdir/file1 | _filter_scratch
> @@ -102,10 +117,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
>  
> -echo "Seek holes and data in file1"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
> -echo "Seek holes and data in file2"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
> +exercise_lseek
>  
>  echo "Compare files"
>  md5sum $testdir/file1 | _filter_scratch
> @@ -121,10 +133,7 @@ $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file1 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file2 >> $seqres.full 2>&1
>  $XFS_IO_PROG -c "bmap -ev" -c "bmap -cv" $testdir/file3 >> $seqres.full 2>&1
>  
> -echo "Seek holes and data in file1"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file1
> -echo "Seek holes and data in file2"
> -$XFS_IO_PROG -c "seek -a -r 0" $testdir/file2
> +exercise_lseek
>  
>  echo "Compare files"
>  md5sum $testdir/file1 | _filter_scratch
>