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Date: Wed, 5 Jun 2024 15:26:11 +0100
From: John Garry <john.g.garry@...cle.com>
To: david@...morbit.com, djwong@...nel.org, hch@....de,
viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
chandan.babu@...cle.com, willy@...radead.org
Cc: axboe@...nel.dk, martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
ojaswin@...ux.ibm.com, ritesh.list@...il.com, mcgrof@...nel.org,
p.raghav@...sung.com, linux-xfs@...r.kernel.org,
catherine.hoang@...cle.com, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH v3 07/21] fs: xfs: align args->minlen for forced
allocation alignment
On 29/04/2024 18:47, John Garry wrote:
> From: Dave Chinner <dchinner@...hat.com>
>
> If args->minlen is not aligned to the constraints of forced
> alignment, we may do minlen allocations that are not aligned when we
> approach ENOSPC. Avoid this by always aligning args->minlen
> appropriately. If alignment of minlen results in a value smaller
> than the alignment constraint, fail the allocation immediately.
>
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
> fs/xfs/libxfs/xfs_bmap.c | 45 +++++++++++++++++++++++++++-------------
> 1 file changed, 31 insertions(+), 14 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 7a0ef0900097..4f39a43d78a7 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3288,33 +3288,48 @@ xfs_bmap_longest_free_extent(
> return 0;
> }
>
> -static xfs_extlen_t
> +static int
> xfs_bmap_select_minlen(
> struct xfs_bmalloca *ap,
> struct xfs_alloc_arg *args,
> xfs_extlen_t blen)
> {
> -
> /* Adjust best length for extent start alignment. */
> if (blen > args->alignment)
> blen -= args->alignment;
>
> /*
> * Since we used XFS_ALLOC_FLAG_TRYLOCK in _longest_free_extent(), it is
> - * possible that there is enough contiguous free space for this request.
> + * possible that there is enough contiguous free space for this request
> + * even if best length is less that the minimum length we need.
> + *
> + * If the best length won't satisfy the maximum length we requested,
> + * then use it as the minimum length so we get as large an allocation
> + * as possible.
> */
> if (blen < ap->minlen)
> - return ap->minlen;
> + blen = ap->minlen;
> + else if (blen > args->maxlen)
> + blen = args->maxlen;
>
> /*
> - * If the best seen length is less than the request length,
> - * use the best as the minimum, otherwise we've got the maxlen we
> - * were asked for.
> + * If we have alignment constraints, round the minlen down to match the
> + * constraint so that alignment will be attempted. This may reduce the
> + * allocation to smaller than was requested, so clamp the minimum to
> + * ap->minlen to allow unaligned allocation to succeed. If we are forced
> + * to align the allocation, return ENOSPC at this point because we don't
> + * have enough contiguous free space to guarantee aligned allocation.
> */
> - if (blen < args->maxlen)
> - return blen;
> - return args->maxlen;
> -
> + if (args->alignment > 1) {
> + blen = rounddown(blen, args->alignment);
> + if (blen < ap->minlen) {
> + if (args->datatype & XFS_ALLOC_FORCEALIGN)
> + return -ENOSPC;
> + blen = ap->minlen;
> + }
> + }
Hi Dave,
I still think that there is a problem with this code or some other
allocator code which gives rise to unexpected -ENOSPC. I just highlight
this code, above, as I get an unexpected -ENOSPC failure here when the
fs does have many free (big enough) extents. I think that the problem
may be elsewhere, though.
Initially we have a file like this:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
0: [0..127]: 62592..62719 0 (62592..62719) 128
1: [128..895]: hole 768
2: [896..1023]: 63616..63743 0 (63616..63743) 128
3: [1024..1151]: 64896..65023 0 (64896..65023) 128
4: [1152..1279]: 65664..65791 0 (65664..65791) 128
5: [1280..1407]: 68224..68351 0 (68224..68351) 128
6: [1408..1535]: 76416..76543 0 (76416..76543) 128
7: [1536..1791]: 62720..62975 0 (62720..62975) 256
8: [1792..1919]: 60032..60159 0 (60032..60159) 128
9: [1920..2047]: 63488..63615 0 (63488..63615) 128
10: [2048..2303]: 63744..63999 0 (63744..63999) 256
forcealign extsize is 16 4k fsb, so the layout looks ok.
Then we truncate the file to 454 sectors (or 56.75 fsb). This gives:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
0: [0..127]: 62592..62719 0 (62592..62719) 128
1: [128..455]: hole 328
We have 57 fsb.
Then I attempt to write from byte offset 232448 (454 sector) and a get a
write failure in xfs_bmap_select_minlen() returning -ENOSPC; at that
point the file looks like this:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
0: [0..127]: 62592..62719 0 (62592..62719) 128
1: [128..447]: hole 320
2: [448..575]: 62720..62847 0 (62720..62847) 128
That hole in ext #1 is 40 fsb, and not aligned with forcealign
granularity. This means that ext #2 is misaligned wrt forcealign
granularity.
This is strange.
I notice that we when allocate ext #2, xfs_bmap_btalloc() returns
ap->blkno=7840, length=16, offset=56. I would expect offset % 16 == 0,
which it is not.
In the following sub-io block zeroing, I note that we zero the front
padding from pos=196608 (or fsb 48 or sector 384) for len=35840, and
back padding from pos=263680 for len=64000 (upto sector 640 or fsb 80).
That seems wrong, as we are zeroing data in the ext #1 hole, right?
Now the actual -ENOSPC comes from xfs_bmap_btalloc() -> ... ->
xfs_bmap_select_minlen() with initially blen=32 args->alignment=16
ap->minlen=1 args->maxlen=8. There xfs_bmap_btalloc() has ap->length=8
initially. This may be just a symptom.
With args->maxlen < args->alignment, we fail with -ENOSPC in
xfs_bmap_select_minlen()
I guess that there is something wrong in the block allocator for ext #2.
Any idea where to check?
I'll send a new v4 series soon which has this problem, as to share the
exact full code changes.
Thanks,
John
> + args->minlen = blen;
> + return 0;
> }
>
> static int
> @@ -3350,8 +3365,7 @@ xfs_bmap_btalloc_select_lengths(
> if (pag)
> xfs_perag_rele(pag);
>
> - args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> - return error;
> + return xfs_bmap_select_minlen(ap, args, blen);
> }
>
> /* Update all inode and quota accounting for the allocation we just did. */
> @@ -3671,7 +3685,10 @@ xfs_bmap_btalloc_filestreams(
> goto out_low_space;
> }
>
> - args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> + error = xfs_bmap_select_minlen(ap, args, blen);
> + if (error)
> + goto out_low_space;
> +
> if (ap->aeof && ap->offset)
> error = xfs_bmap_btalloc_at_eof(ap, args);
>
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