Re: [External] : [PATCH 1/2] xfs: set minleft correctly for sparse chunk errortag allocation

On Fri, Jul 31, 2026 at 01:02:50PM -0500, Mark Tinguely wrote:
> On 7/31/26 11:33 AM, Brian Foster wrote:
> > The errortag instrumentation for forced sparse chunk allocation
> > jumps straight to the allocation path without setting args.minleft.
> > minleft is unconditionally set to ->inobt_maxlevels for the normal
> > allocation path. Lift the assignment to the initial args setup so
> > it covers all possible paths.
> > 
> > Assisted-by: LLM
> > Fixes: 1cdadee11f8d ("xfs: randomly do sparse inode allocations in DEBUG mode")
> > Signed-off-by: Brian Foster <[email protected]>
> > ---
> >   fs/xfs/libxfs/xfs_ialloc.c | 10 ++++------
> >   1 file changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> > index ffcdd1f691fd..633b2d6e42c5 100644
> > --- a/fs/xfs/libxfs/xfs_ialloc.c
> > +++ b/fs/xfs/libxfs/xfs_ialloc.c
> > @@ -733,6 +733,10 @@ xfs_ialloc_ag_alloc(
> >   							igeo->maxicount)
> >   		return -ENOSPC;
> >   	args.minlen = args.maxlen = igeo->ialloc_blks;
> > +
> > +	/* Allow space for the inode btree to split. */
> > +	args.minleft = igeo->inobt_maxlevels;
> > +
> >   	/*
> >   	 * First try to allocate inodes contiguous with the last-allocated
> >   	 * chunk of inodes.  If the filesystem is striped, this will fill
> > @@ -764,8 +768,6 @@ xfs_ialloc_ag_alloc(
> >   		args.alignment = 1;
> >   		args.minalignslop = igeo->cluster_align - 1;
> > -		/* Allow space for the inode btree to split. */
> > -		args.minleft = igeo->inobt_maxlevels;
> >   		error = xfs_alloc_vextent_exact_bno(&args,
> >   				xfs_agbno_to_fsb(pag, args.agbno));
> >   		if (error)
> > @@ -804,10 +806,6 @@ xfs_ialloc_ag_alloc(
> >   		 * Allocate a fixed-size extent of inodes.
> >   		 */
> >   		args.prod = 1;
> > -		/*
> > -		 * Allow space for the inode btree to split.
> > -		 */
> > -		args.minleft = igeo->inobt_maxlevels;
> >   		error = xfs_alloc_vextent_near_bno(&args,
> >   				xfs_agbno_to_fsb(pag,
> >   					be32_to_cpu(agi->agi_root)));
> 
> 
> Reviewed-by: Mark Tinguely <[email protected]>
> 

Thanks..

> I got the patches backwards...I agree we need to increase the minleft in
> the other patch by 4 blocks (2 btrees * 2 for the level increase) but I
> wonder why can't a full size inode chunk also split the by-count and
> by-block btrees and the inode btree allocation also be short when fixing AGFL?
> 

Yeah, I had the same general question in the original report. I'm not
sure it can't happen for full size chunks. I was kind of taking it for
granted that sparse inodes seem required to reproduce this just by
virtue of smaller allocations and the allocator maybe having enough slop
in the various calculations to gate it otherwise, but I'm not confident
in that having now looked more through this code.

Perhaps Matt can comment on if or how hard we tried to reproduce this
with full inode chunks..? I'm curious.. was the creation of the
reproducer based on a detected flaw in the code (LLM scan or
something?), or a low space stress test or something were this problem
just happened to fall out..?

If we could reproduce or manufacture this with normal inode chunks, then
I suppose that would lend more credence to a more generic solution
within the allocator.

Brian