Re: [BUG] xfs: sparse inode allocation can trip i != 1 after AGFL growth

[Matt Fleming <matt@readmodwrite.com>]

On Fri, Jul 17, 2026 at 01:55:42PM -0400, Brian Foster wrote:
> On Fri, Jul 17, 2026 at 02:04:29PM +0100, Matt Fleming wrote:
> > 
> > The failure sequence seems to be:
> > 
> > 1. AG0 has no free inodes, so inode allocation needs a new chunk.
> > 2. Full chunk allocation cannot fit.
> > 3. Sparse chunk allocation can fit and passes the old AGFL minimum check.
> > 4. Removing the sparse extent from free space grows both bnobt and cntbt.
> 
> I assume this means we happen to alloc a sparse chunk out of the middle
> of a free extent, causing an additional record and thus splits in both
> trees.
 
Yep, exactly.

> It also looks like we set args.minleft = igeo->inobt_maxlevels in the
> chunk alloc path, presumably with the intent to leave enough blocks
> around for inode record insertion. I assume the above available value is
> prior to the sparse chunk alloc. It would be interesting to see what the
> same values/calculations are after the chunk alloc at the time of the
> inobt block alloc that fails.
 
Before sparse inode allocation:

   pagf_freeblks:    2514
   pagf_flcount:     8
   levels:           bno/cnt/rmap = 1/1/2
   reservation:      2505
   minleft:          2
   min_freelist:     8
   AGFL credit:      min(8, 8) = 8

   available:
     2514 + 8 - 2505 - 8 - 2 = 7

   request:
     minlen=4 align=4 slop=0
     alloc_len = 4 + (4 - 1) + 0 = 7

So the sparse inode allocation only just passes the allocator check.

After sparse inode allocation, at the failing inobt split allocation:

   pagf_freeblks:    2510
   pagf_flcount:     4
   levels:           bno/cnt/rmap = 2/2/2
   reservation:      2505
   minleft:          0
   min_freelist:     12
   AGFL credit:      min(4, 12) = 4

   available:
     2510 + 4 - 2505 - 12 - 0 = -3

   request:
     minlen=1

So after removing the 4-block sparse extent, bnobt/cntbt have grown,
AGFL has dropped, min_freelist has increased from 8 to 12, and the
later single-block inobt allocation has no available space.

Thanks,
Matt