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

[John Garry <john.g.garry@oracle.com>]

On 29/04/2024 18:47, John Garry wrote:
> From: Dave Chinner <dchinner@redhat.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@redhat.com>
> Signed-off-by: John Garry <john.g.garry@oracle.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);
>