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

[Dave Chinner <david@fromorbit.com>]

On Fri, Apr 17, 2015 at 08:28:29AM +1000, Dave Chinner wrote:
> On Thu, Apr 16, 2015 at 01:32:38PM -0400, Brian Foster wrote:
> > On Thu, Apr 16, 2015 at 03:00:50PM +1000, Dave Chinner wrote:
> > > + * 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):
> 
> But I assumed the existing code was correct for this context. My
> bad. :/
> 
> > $ 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
> 
> Yup, because it 2^21 = 2G, and extsize = 1g puts max_length < 0.
> 
> Ok. So, the problem is that it is overestimating the amount of space
> that alignment will need, and that alignment cannot be guaranteed
> for extsz hints of over (MAXEXTLEN / 2) in size.
> 
> i.e. given an alignment (A[0-2]) and an extent (E[01]):
> 
>    A0                  A1                  A2
>    +-------------------+-------------------+
> 		     +ooo+
> 		     E0  E1
> 
> The problem is that the alignment done by xfs_bmap_extsize_align()
> only extends outwards (i.e. increases extent size). Hence E0 gets
> rounded down to A0-A2, and E1 gets extended to A2, which means we
> are adding almost 2 entire extent size hints to the allocation.
> That's where the reduction in length by two extsz values came from.
> 
> Now, for delayed allocation, this is just fine, because real
> allocation will break this delalloc extent up into two separate
> extents, and underflow wouldn't be noticed as delalloc extents are
> not physically limited to MAXEXTLEN and so nothing would have
> broken. Still, it's not the intended behaviour.
> 
> I'm not sure what the solution is yet - the fundamental problem here
> is the outwards alignment of both ends of the extent, and this
> MAXEXTLEN twiddling is just an encoding of that behaviour. I need to
> spend some time looking at xfs_bmap_extsize_align() and determining
> if there is something we can do differently here.

Ok, so the callers of xfs_bmap_extsize_align() are:

	xfs_bmapi_reserve_delalloc()
	xfs_bmap_btalloc()
	xfs_bmap_rtalloc().

For xfs_bmapi_reserve_delalloc(), the alignment does not need grow
outwards; it can be truncated mid-range, and the code should still
work. i.e.

   A0                  A1                  A2
   +-------------------+-------------------+
		     +ooo+
		     E0  E1
   +-------------------+
   R0                  R1

R[01] is a valid alignment and will result in a second allocation
occurring for this:

   A0                  A1                  A2
   +-------------------+-------------------+
		       +o+
		      E2  E1
                       +-------------------+
                       R1                  R2

And so the range we need allocation for (E[01]) will be allocated
and correctly extent size aligned.

For xfs_bmap_btalloc() - the problem case here - the code is a
little more complex. We do:

xfs_bmapi_write
  loop until all allocated {
    xfs_bmapi_allocate(bma)
      calc off/len
        xfs_bmap_btalloc(bma)
	  xfs_bmap_extsize_align(bma)
	  xfs_alloc_vextent
	  update bma->length
      BMBT insert
    trim returned map
  }

So we are doing alignment two steps removed from the off/len
calculation (xfs_bmap_rtalloc() is in the same boat). Hence the
question is whether xfs_bmap_extsize_align() can trim the range
being allocated and still have everything work....

Ok, upon further reading, the xfs_bmalloc structure (bma) that is
passed between these functions to track the allocation being done is
updated after allocation with the length of the extent allocated.
IOWs:

	bma->length = E(len)
	xfs_bmap_btalloc(bma)
	  A(len) = xfs_bmap_extsize_align(bma->length)
	  R(len) = xfs_alloc_vextent(A(len))
	  bma->length = R(len)

Hence this:

   A0                  A1                  A2
   +-------------------+-------------------+
		     +ooo+
		     E0  E1
   +-------------------+
   R0                  R1

Is a valid result from xfs_bmap_btalloc() and the loop in
xfs_bmapi_write() will do a second allocation and alignment as per
the above delalloc case. xfs_bmap_rtalloc() appears to mirror this
same structure, so should also have the same behaviour.

What this means is that we can actually reduce the requested
allocation to be only a partial overlap when aligning it, and
everything should still work. Let's now see how complex that makes
the code...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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