Re: [PATCH] xfs: fix off-by-one in inode alloc block reservation calculation

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Aug 20, 2020 at 01:07:34PM -0400, Brian Foster wrote:
> The inode chunk allocation transaction reserves inobt_maxlevels-1
> blocks to accommodate a full split of the inode btree. A full split
> requires an allocation for every existing level and a new root
> block, which means inobt_maxlevels is the worst case block
> requirement for a transaction that inserts to the inobt. This can
> lead to a transaction block reservation overrun when tmpfile
> creation allocates an inode chunk and expands the inobt to its
> maximum depth. This problem has been observed in conjunction with
> overlayfs, which makes frequent use of tmpfiles internally.
> 
> The existing reservation code goes back as far as the Linux git repo
> history (v2.6.12). It was likely never observed as a problem because
> the traditional file/directory creation transactions also include
> worst case block reservation for directory modifications, which most
> likely is able to make up for a single block deficiency in the inode
> allocation portion of the calculation. tmpfile support is relatively
> more recent (v3.15), less heavily used, and only includes the inode
> allocation block reservation as tmpfiles aren't linked into the
> directory tree on creation.
> 
> Fix up the inode alloc block reservation macro and a couple of the
> block allocator minleft parameters that enforce an allocation to
> leave enough free blocks in the AG for a full inobt split.

Looks all fine to me, but... does a similar logic apply to the other
maxlevels uses in the kernel?

fs/xfs/libxfs/xfs_trans_resv.c:73:      blocks = num_ops * 2 * (2 * mp->m_ag_maxlevels - 1);
fs/xfs/libxfs/xfs_trans_resv.c:75:              blocks += max(num_ops * (2 * mp->m_rmap_maxlevels - 1),
fs/xfs/libxfs/xfs_trans_resv.c:78:              blocks += num_ops * (2 * mp->m_refc_maxlevels - 1);

Can we end up in the same kind of situation with those other trees
{bno,cnt,rmap,refc} where we have a maxlevels-1 tall tree and split each
level all the way to the top?

> Signed-off-by: Brian Foster <bfoster@redhat.com>

For this bit,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/xfs/libxfs/xfs_ialloc.c      | 4 ++--
>  fs/xfs/libxfs/xfs_trans_space.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index f742a96a2fe1..a6b37db55169 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -688,7 +688,7 @@ xfs_ialloc_ag_alloc(
>  		args.minalignslop = igeo->cluster_align - 1;
>  
>  		/* Allow space for the inode btree to split. */
> -		args.minleft = igeo->inobt_maxlevels - 1;
> +		args.minleft = igeo->inobt_maxlevels;
>  		if ((error = xfs_alloc_vextent(&args)))
>  			return error;
>  
> @@ -736,7 +736,7 @@ xfs_ialloc_ag_alloc(
>  		/*
>  		 * Allow space for the inode btree to split.
>  		 */
> -		args.minleft = igeo->inobt_maxlevels - 1;
> +		args.minleft = igeo->inobt_maxlevels;
>  		if ((error = xfs_alloc_vextent(&args)))
>  			return error;
>  	}
> diff --git a/fs/xfs/libxfs/xfs_trans_space.h b/fs/xfs/libxfs/xfs_trans_space.h
> index c6df01a2a158..7ad3659c5d2a 100644
> --- a/fs/xfs/libxfs/xfs_trans_space.h
> +++ b/fs/xfs/libxfs/xfs_trans_space.h
> @@ -58,7 +58,7 @@
>  #define	XFS_IALLOC_SPACE_RES(mp)	\
>  	(M_IGEO(mp)->ialloc_blks + \
>  	 ((xfs_sb_version_hasfinobt(&mp->m_sb) ? 2 : 1) * \
> -	  (M_IGEO(mp)->inobt_maxlevels - 1)))
> +	  M_IGEO(mp)->inobt_maxlevels))
>  
>  /*
>   * Space reservation values for various transactions.
> -- 
> 2.25.4
>