Re: [PATCH] xfs: fix inode allocation block res calculation precedence

[Dave Chinner <david@fromorbit.com>]

On Wed, Jul 15, 2020 at 03:33:10PM -0400, Brian Foster wrote:
> The block reservation calculation for inode allocation is supposed
> to consist of the blocks required for the inode chunk plus
> (maxlevels-1) of the inode btree multiplied by the number of inode
> btrees in the fs (2 when finobt is enabled, 1 otherwise).
> 
> Instead, the macro returns (ialloc_blocks + 2) due to a precedence
> error in the calculation logic. This leads to block reservation
> overruns via generic/531 on small block filesystems with finobt
> enabled. Add braces to fix the calculation and reserve the
> appropriate number of blocks.
> 
> Fixes: 9d43b180af67 ("xfs: update inode allocation/free transaction reservations for finobt")
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_trans_space.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_trans_space.h b/fs/xfs/libxfs/xfs_trans_space.h
> index 88221c7a04cc..c6df01a2a158 100644
> --- a/fs/xfs/libxfs/xfs_trans_space.h
> +++ b/fs/xfs/libxfs/xfs_trans_space.h
> @@ -57,7 +57,7 @@
>  	XFS_DAREMOVE_SPACE_RES(mp, XFS_DATA_FORK)
>  #define	XFS_IALLOC_SPACE_RES(mp)	\
>  	(M_IGEO(mp)->ialloc_blks + \
> -	 (xfs_sb_version_hasfinobt(&mp->m_sb) ? 2 : 1 * \
> +	 ((xfs_sb_version_hasfinobt(&mp->m_sb) ? 2 : 1) * \
>  	  (M_IGEO(mp)->inobt_maxlevels - 1)))

Ugh. THese macros really need rewriting as static inline functions.
This would not have happened if it were written as:

static inline int
xfs_ialloc_space_res(struct xfs_mount *mp)
{
	int	res = M_IGEO(mp)->ialloc_blks;

	res += M_IGEO(mp)->inobt_maxlevels - 1;
	if (xfs_sb_version_hasfinobt(&mp->m_sb))
		res += M_IGEO(mp)->inobt_maxlevels - 1;
	return res;
}

Next question: why is this even a macro that is calculated on demand
instead of a read-only constant held in inode geometry calculated
at mount time? Then it doesn't even need to be an inline function
and can just be rolled into xfs_ialloc_setup_geometry()....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com