Re: [PATCH 1/8] xfs/172: disable test when file writes don't use delayed allocation

[Allison Henderson <allison.henderson@oracle.com>]

On 7/6/21 5:21 PM, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> This test tries to exploit an interaction between delayed allocation and
> writeback on full filesystems to see if it can trip up the filestreams
> allocator.  The behaviors do not present if the filesystem allocates
> space at write time, so disable it under these scenarios.
Ok, makes sense
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>

> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>   tests/xfs/172 |   30 +++++++++++++++++++++++++++++-
>   1 file changed, 29 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/tests/xfs/172 b/tests/xfs/172
> index 0d1b441e..c0495305 100755
> --- a/tests/xfs/172
> +++ b/tests/xfs/172
> @@ -16,9 +16,37 @@ _begin_fstest rw filestreams
>   
>   # real QA test starts here
>   _supported_fs xfs
> -
> +_require_command "$FILEFRAG_PROG" filefrag
>   _require_scratch
>   
> +# The first _test_streams call sets up the filestreams allocator to fail and
> +# then checks that it actually failed.  It does this by creating a very small
> +# filesystem, writing a lot of data in parallel to separate streams, and then
> +# flushes the dirty data, also in parallel.  To trip the allocator, the test
> +# relies on writeback combining adjacent dirty ranges into large allocation
> +# requests which eventually bleed across AGs.  This happens either because the
> +# big writes are slow enough that filestreams contexts expire between
> +# allocation requests, or because the AGs are so full at allocation time that
> +# the bmapi allocator decides to scan for a less full AG.  Either way, stream
> +# directories share AGs, which is what the test considers a success.
> +#
> +# However, this only happens if writes use the delayed allocation code paths.
> +# If the kernel allocates small amounts of space at the time of each write()
> +# call, the successive small allocations never trip the bmapi allocator's
> +# rescan thresholds and will keep pushing out the expiration time, with the
> +# result that the filestreams allocator succeeds in maintaining the streams.
> +# The test considers this a failure.
> +#
> +# Make sure that a regular buffered write produces delalloc reservations.
> +# This effectively disables the test for files with extent size hints or DAX
> +# mode set.
> +_scratch_mkfs > $seqres.full
> +_scratch_mount
> +$XFS_IO_PROG -f -c 'pwrite 0 64k' $SCRATCH_MNT/testy &> /dev/null
> +$FILEFRAG_PROG -v $SCRATCH_MNT/testy 2>&1 | grep -q delalloc || \
> +	_notrun "test requires delayed allocation buffered writes"
> +_scratch_unmount
> +
>   _check_filestreams_support || _notrun "filestreams not available"
>   
>   # test reaper works by setting timeout low. Expected to fail
>