Re: [PATCH RFC] generic 223: Ensure xfs allocator will honor alignment requirements

[Brian Foster <bfoster@redhat.com>]

On Mon, Aug 26, 2019 at 04:47:12PM +0200, Carlos Maiolino wrote:
> If the files being allocated during the test do not fit into a single
> Allocation Group, XFS allocator may disable alignment requirements
> causing the test to fail even though XFS was working as expected.
> 
> Fix this by fixing a min AG size, so all files created during the test
> will fit into a single AG not disabling XFS alignment requirements.
> 
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
> 
> Hi,
> 
> I am tagging this patch as a RFC mostly to start a discussion here, regarding
> this issue found while running generic/223.
> 
> The generic/223 fails when running it with finobt disabled. Specifically, the
> last file being fallocated are unaligned.
> 
> When the finobt is enabled, the allocator does not try to squeeze partial file
> data into small available extents in AG 0, while it does when finobt is
> disabled.
> 
> Here are the bmap of the same file after generic/223 finishes with and without
> finobt:
> 
> finobt=0
> 
> /mnt/scratch/file-1073745920-falloc:
>  EXT: FILE-OFFSET         BLOCK-RANGE      AG AG-OFFSET           TOTAL FLAGS
>    0: [0..191]:           320..511          0 (320..511)            192 001011
>    1: [192..375]:         64..247           0 (64..247)             184 001111
>    2: [376..1287791]:     678400..1965815   0 (678400..1965815) 1287416 000111
>    3: [1287792..2097159]: 1966080..2775447  1 (256..809623)      809368 000101
> 
> 
> finobt=1
> 
> /mnt/scratch/file-1073745920-falloc:
>  EXT: FILE-OFFSET         BLOCK-RANGE      AG AG-OFFSET           TOTAL FLAGS
>    0: [0..1285831]:       678400..1964231   0 (678400..1964231) 1285832 000111
>    1: [1285832..2097159]: 1966080..2777407  1 (256..811583)      811328 000101
> 
> 
> I still don't know the details about why the allocator takes different decisions
> depending on finobt being used or not, although I believe it's because the extra
> space being used in each AG, and the default AG size when running the test, but
> I'm still reading the code to try to understand this difference.
> 

For reference, I think this behavior is related to a couple patches I
posted a few months ago[1]. I reproduced similar behavior after some of
extent allocation rework changes and ultimately determined that the
changes I had at the time weren't really the root cause. The commit log
for patch 1 in that series shows a straightforward example that IIRC
doesn't have anything to do with finobt either.

> Even though I think there might be room for improvement in the XFS allocator
> code to avoid this bypass of alignment requirements here, I still think the test
> should be fixed to avoid forcing the filesystem to drop alignment constraints
> during file allocation which basically invalidate the test, and that's why I
> decided to start the discussion with a RFC patch for the test, but sending it to
> xfs list instead of fstests.
> 

The question I have is is this test doing anything a user wouldn't
expect to honor alignment? I understand that alignment is not
guaranteed, but I wouldn't expect to play that card unless the
filesystem is low on free space or aligned space in general (IIRC that's
something we check by adding the worst case alignment to the size of the
allocation request).

The example I had was a 1GB fallocate on an empty ~15GB/16AG fs (i.e.
allocation larger than a single AG). That is a corner case, but one I'd
expect to work. With regard to generic/223, what is it doing in this
case to "force the filesystem to drop alignment?" I think you could make
the argument that the test needs fixing if it's doing something that
legitimately risks the ability to align allocations, but I also think
you could argue that the test has done its job by finding this problem.
:)

Brian

[1] https://marc.info/?l=linux-xfs&m=155671950608062&w=2

> Comments?
> 
> Cheers
> 
> 
>  tests/generic/223 | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/tests/generic/223 b/tests/generic/223
> index dfd8c41b..782651e2 100755
> --- a/tests/generic/223
> +++ b/tests/generic/223
> @@ -34,6 +34,13 @@ _require_xfs_io_command "falloc"
>  
>  rm -f $seqres.full
>  
> +# Ensure we won't trick xfs allocator to disable alignment requirements
> +if [ "$FSTYP" == "xfs" ]; then
> +	mkfs_opts="-d agsize=2g"
> +else
> +	mkfs_opts=""
> +fi
> +
>  BLOCKSIZE=4096
>  
>  for SUNIT_K in 8 16 32 64 128; do
> @@ -41,7 +48,7 @@ for SUNIT_K in 8 16 32 64 128; do
>  	let SUNIT_BLOCKS=$SUNIT_BYTES/$BLOCKSIZE
>  
>  	echo "=== mkfs with su $SUNIT_BLOCKS blocks x 4 ==="
> -	export MKFS_OPTIONS=""
> +	export MKFS_OPTIONS=$mkfs_opts
>  	_scratch_mkfs_geom $SUNIT_BYTES 4 $BLOCKSIZE >> $seqres.full 2>&1
>  	_scratch_mount
>  
> -- 
> 2.20.1
>