Re: [PATCH v2] xfs: extent size hints can round up extents past MAXEXTLEN

[Brian Foster <bfoster@redhat.com>]

On Thu, Apr 16, 2015 at 03:00:50PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> This results in BMBT corruption, as seen by this test:
> 
> # mkfs.xfs -f -d size=40051712b,agcount=4 /dev/vdc
> ....
> # mount /dev/vdc /mnt/scratch
> # xfs_io -ft -c "extsize 16m" -c "falloc 0 30g" -c "bmap -vp" /mnt/scratch/foo
> 
> which results in this failure on a debug kernel:
> 
> XFS: Assertion failed: (blockcount & xfs_mask64hi(64-BMBT_BLOCKCOUNT_BITLEN)) == 0, file: fs/xfs/libxfs/xfs_bmap_btree.c, line: 211
> ....
> Call Trace:
>  [<ffffffff814cf0ff>] xfs_bmbt_set_allf+0x8f/0x100
>  [<ffffffff814cf18d>] xfs_bmbt_set_all+0x1d/0x20
>  [<ffffffff814f2efe>] xfs_iext_insert+0x9e/0x120
>  [<ffffffff814c7956>] ? xfs_bmap_add_extent_hole_real+0x1c6/0xc70
>  [<ffffffff814c7956>] xfs_bmap_add_extent_hole_real+0x1c6/0xc70
>  [<ffffffff814caaab>] xfs_bmapi_write+0x72b/0xed0
>  [<ffffffff811c72ac>] ? kmem_cache_alloc+0x15c/0x170
>  [<ffffffff814fe070>] xfs_alloc_file_space+0x160/0x400
>  [<ffffffff81ddcc29>] ? down_write+0x29/0x60
>  [<ffffffff815063eb>] xfs_file_fallocate+0x29b/0x310
>  [<ffffffff811d2bc8>] ? __sb_start_write+0x58/0x120
>  [<ffffffff811e3e18>] ? do_vfs_ioctl+0x318/0x570
>  [<ffffffff811cd680>] vfs_fallocate+0x140/0x260
>  [<ffffffff811ce6f8>] SyS_fallocate+0x48/0x80
>  [<ffffffff81ddec09>] system_call_fastpath+0x12/0x17
> 
> The tracepoint that indicates the extent that triggered the assert
> failure is:
> 
> xfs_iext_insert:   idx 0 offset 0 block 16777224 count 2097152 flag 1
> 
> Clearly indicating that the extent length is greater than MAXEXTLEN,
> which is 2097151. A prior trace point shows the allocation was an
> exact size match and that a length greater than MAXEXTLEN was asked
> for:
> 
> xfs_alloc_size_done:  agno 1 agbno 8 minlen 2097152 maxlen 2097152
> 					    ^^^^^^^        ^^^^^^^
> 
> The issue is that the extent size hint alignment is rounding up the
> extent size past MAXEXTLEN, because xfs_bmapi_write() is not taking
> into account extent size hints when calculating the maximum extent
> length to allocate. xfs_bmapi_reserve_delalloc() is already doing
> this, but direct extent allocation is not.
> 
> We don't see this problem with extent size hints through the IO path
> because we can't do single IOs large enough to trigger MAXEXTLEN
> allocation. fallocate(), OTOH, is not limited in it's allocation
> sizes and so needs help here. The fix is simply to copy the logic
> from xfs_bmapi_reserve_delalloc() and apply it apropriately to
> xfs_bmapi_write().
> 
> I also add an ASSERT() to xfs_bmap_extsize_align() so we'll catch
> cases of alignment exceeding MAXEXTLEN on debug kernel machines in
> future.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Looks fine from the perspective of applying pre-existing logic to a
separate codepath, but...

>  fs/xfs/libxfs/xfs_bmap.c | 51 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 37 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index aeffeaa..37949b5 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3224,12 +3224,21 @@ xfs_bmap_extsize_align(
>  		align_alen += temp;
>  		align_off -= temp;
>  	}
> +
> +	/* Same adjustment for the end of the requested area. */
> +	temp = (align_alen % extsz);
> +	if (temp)
> +		align_alen += extsz - temp;
> +
>  	/*
> -	 * Same adjustment for the end of the requested area.
> +	 * we are in trouble if the caller requested an extent that will align
> +	 * to something larger than the supported on disk extent size.  Assert
> +	 * fail here to catch callers that make this mistake; they should always
> +	 * be setting the maximum allocation length to be (MAXEXTLEN - extsz) so
> +	 * we can round outwards here for alignment.
>  	 */
> -	if ((temp = (align_alen % extsz))) {
> -		align_alen += extsz - temp;
> -	}
> +	ASSERT(align_alen <= MAXEXTLEN);
> +
>  	/*
>  	 * If the previous block overlaps with this proposed allocation
>  	 * then move the start forward without adjusting the length.
> @@ -4074,6 +4083,27 @@ xfs_bmapi_read(
>  	return 0;
>  }
>  
> +/*
> + * Calculate the maximum extent length we can ask to allocate after taking into
> + * account the on-disk size limitations, the extent size hints and the size
> + * being requested. We have to deal with the extent size hint here because the
> + * allocation will attempt alignment and hence grow the length outwards by up to
> + * @extsz on either side.
> + */
> +static inline xfs_extlen_t
> +xfs_bmapi_max_extlen(
> +	struct xfs_inode	*ip,
> +	xfs_extlen_t		length)
> +{
> +	xfs_extlen_t		extsz = xfs_get_extsz_hint(ip);
> +	xfs_extlen_t		max_length = MAXEXTLEN;
> +
> +	if (extsz)
> +		max_length -= 2 * extsz - 1;

This can underflow or cause other issues if set to just the right value
(with smaller block sizes such that length can be trimmed to 0):

$ mkfs.xfs -f -bsize=1k <dev>
$ mount <dev> /mnt
$ xfs_io -f -c "extsize 1g" -c "pwrite 0 4k" -c fsync /mnt/file
pwrite64: No space left on device

...
 kernel:XFS: Assertion failed: ap->length, file: fs/xfs/libxfs/xfs_bmap.c, line: 3652
...

$ xfs_io -f -c "extsize 1025m" -c "pwrite 0 4k" -c fsync /mnt/file
wrote 4096/4096 bytes at offset 0
4 KiB, 4 ops; 0.0000 sec (1.212 MiB/sec and 1241.0797 ops/sec)

(Both that and the original reproducer might make a good xfstests test,
btw...)

Brian

> +
> +	return (length < max_length) ? length : max_length;
> +}
> +
>  STATIC int
>  xfs_bmapi_reserve_delalloc(
>  	struct xfs_inode	*ip,
> @@ -4092,20 +4122,13 @@ xfs_bmapi_reserve_delalloc(
>  	xfs_extlen_t		extsz;
>  	int			error;
>  
> -	alen = XFS_FILBLKS_MIN(len, MAXEXTLEN);
> +	alen = xfs_bmapi_max_extlen(ip, len);
>  	if (!eof)
>  		alen = XFS_FILBLKS_MIN(alen, got->br_startoff - aoff);
>  
> -	/* Figure out the extent size, adjust alen */
> +	/* Figure out the extent size, align alen */
>  	extsz = xfs_get_extsz_hint(ip);
>  	if (extsz) {
> -		/*
> -		 * Make sure we don't exceed a single extent length when we
> -		 * align the extent by reducing length we are going to
> -		 * allocate by the maximum amount extent size aligment may
> -		 * require.
> -		 */
> -		alen = XFS_FILBLKS_MIN(len, MAXEXTLEN - (2 * extsz - 1));
>  		error = xfs_bmap_extsize_align(mp, got, prev, extsz, rt, eof,
>  					       1, 0, &aoff, &alen);
>  		ASSERT(!error);
> @@ -4287,7 +4310,7 @@ xfs_bmapi_allocate(
>  					 &bma->prev);
>  		}
>  	} else {
> -		bma->length = XFS_FILBLKS_MIN(bma->length, MAXEXTLEN);
> +		bma->length = xfs_bmapi_max_extlen(bma->ip, bma->length);
>  		if (!bma->eof)
>  			bma->length = XFS_FILBLKS_MIN(bma->length,
>  					bma->got.br_startoff - bma->offset);
> -- 
> 2.0.0
> 
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