Re: [PATCH v3 07/21] fs: xfs: align args->minlen for forced allocation alignment

From: John Garry
Date: Wed Jun 05 2024 - 10:26:56 EST

On 29/04/2024 18:47, John Garry wrote:
From: Dave Chinner <dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
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);