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

[Dave Chinner]

On Wed, Apr 15, 2015 at 04:18:28PM -0500, Eric Sandeen wrote:
> On 4/14/15 7:22 PM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner at 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().
> 
> Cool, thanks for sorting that out!
....
> > @@ -4287,7 +4296,19 @@ xfs_bmapi_allocate(
> >  					 &bma->prev);
> >  		}
> >  	} else {
> > -		bma->length = XFS_FILBLKS_MIN(bma->length, MAXEXTLEN);
> > +		/* Figure out the extent size, adjust alen */
> > +		xfs_extlen_t	maxlen = MAXEXTLEN;
> > +		xfs_extlen_t	extsz = xfs_get_extsz_hint(bma->ip);
> > +
> > +		/*
> > +		 * 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.
> 
> "alignment"  (maybe fix the other comment too?)
> 
> Or better yet, would this be possible to factor into a helper?
> 
> /*              
>  * 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.
> */
> STATIC xfs_extlen_t
> xfs_max_extent_len(
>         struct xfs_inode        *ip)
> {
>         xfs_extlen_t    maxlen = MAXEXTLEN;
>         xfs_extlen_t    extsz = xfs_get_extsz_hint(ip);
> 
> 	/* Insert comment about math here ;) */
>         if (extsz)
>                 maxlen -= (2 * extsz - 1);
> 
>         return maxlen;
> }
> 
> 
> ...
> 
> 	bma->length = XFS_FILBLKS_MIN(bma->length, xfs_max_extent_len(ip));

I thought about that, then just sent the working patch ;)

I'll refactor and send again.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com