Re: [External] : [PATCH 2/2] xfs: consistent low ag space behavior for sparse inode chunk allocs

On 7/31/26 11:33 AM, Brian Foster wrote:
> Matt Fleming reports a filesystem shutdown due to inobt block
> allocation failure during sparse chunk allocation. Inode creation
> can involve multiple allocations in a transaction via the initial
> chunk allocation and inode btree growth via the subsequent inobt
> record insertion. Technically this should be safe as the chunk
> allocation sets the allocation minleft parameter to the max depth of
> the inode btree, which means the allocation selects an AG only if
> there is enough free space for record insertion after the
> allocation. The record insertion naturally occurs in the same AG as
> the allocation and the associated AGF is locked and held by the
> current transaction.
> 
> The situation that reproduces this failure is a bit of a corner
> case. The allocation and inode btrees are all completely full and
> require a split on the next insertion. The AG has just enough
> available space to satisfy inode creation through sparse allocation
> (i.e. 7 blocks in this example). The block allocation occurs within
> an existing free space record, splitting the free space record into
> two and triggering the aforementioned allocbt splits.
> 
> The sparse chunk allocation consumes 4 blocks for the chunk, and 4
> blocks from the AGFL for the two allocbt splits. This leaves the
> AGFL with 4 remaining blocks and 3 available blocks in the AG. The
> record insertion attempts block allocation for the inobt split, but
> even though space is available, the geometry change has changed the
> minimum allocation requirements enforced by
> xfs_alloc_fix_freelist(). The min freelist value jumps from 8 to 12
> due to the alloc btree level increases, so the available space
> calculation goes from something like this before the allocation:
> 
> 	free + agfl - res - minfree - minleft = avail
> 	2514 + 8 - 2505 - 8 - 2 = 7
> 
> ... to this after it:
> 
> 	2510 + 4 - 2505 - 12 - 0 = -3
> 
> Essentially the minleft value of 2 for the chunk alloc is not
> sufficient to overcome the additional requirements imposed on the
> subsequent inobt alloc due to the allocbt geometry changes caused by
> the first.
> 
> Technically this shouldn't be a concern because a full split of any
> of the allocation btrees in one transaction means that we're not
> going to see another full split in the lifetime of that transaction,
> but the allocation path does not behave that way.
> 
> Absent of a clear way to lock in the agfl state for the particular
> case of multiple allocations into a single AG, this patch works
> around the problem by increasing the minleft requirement for sparse
> chunk allocations. Specifically, we increase minleft by the number
> of blocks that the alloc length was reduced by. The reasoning behind
> this is to generally preserve the AG level no space behavior between
> full and sparse inode chunk allocations.
> 
> The purpose of sparse inode chunks is to facilitate allocation in
> AGs under severe free space fragmentation and thus prevent premature
> -ENOSPC conditions across the broader fs, not necessarily squeeze
> every last block out of any single AG. Therefore, this helps ensure
> the low AG space boundary conditions for sparse inode chunks are
> relatively well tested compared to full inode chunks.
> 
> Fixes: 56d1115c9bc7 ("xfs: allocate sparse inode chunks on full chunk allocation failure")
> Reported-by: Matt Fleming <[email protected]>
> Signed-off-by: Brian Foster <[email protected]>
> ---
>   fs/xfs/libxfs/xfs_ialloc.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 633b2d6e42c5..7e71c33fbefa 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -840,6 +840,14 @@ xfs_ialloc_ag_alloc(
>   		args.minlen = igeo->ialloc_min_blks;
>   		args.maxlen = args.minlen;
>   
> +		/*
> +		 * Bump minleft by the alloc size delta to maintain consistent
> +		 * out of space behavior with normal sized chunks. This isn't a
> +		 * requirement, but helps avoid sparse chunk and inobt block
> +		 * allocation quirks at or close to AG depletion.
> +		 */
> +		args.minleft += igeo->ialloc_blks - igeo->ialloc_min_blks;
> +
>   		/*
>   		 * The inode record will be aligned to full chunk size. We must
>   		 * prevent sparse allocation from AG boundaries that result in


Good catch.

Reviewed-by: Mark Tinguely <[email protected]>