Re: [PATCH] xfs: fix inobt inode allocation search optimization

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

On Thu, Aug 10, 2017 at 09:45:45PM -0700, Omar Sandoval wrote:

Looks ok, will test...
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> From: Omar Sandoval <osandov@fb.com>
> 
> When we try to allocate a free inode by searching the inobt, we try to
> find the inode nearest the parent inode by searching chunks both left
> and right of the chunk containing the parent. As an optimization, we
> cache the leftmost and rightmost records that we previously searched; if
> we do another allocation with the same parent inode, we'll pick up the
> search where it last left off.
> 
> There's a bug in the case where we found a free inode to the left of the
> parent's chunk: we need to update the cached left and right records, but
> because we already reassigned the right record to point to the left, we
> end up assigning the left record to both the cached left and right
> records.
> 
> This isn't a correctness problem strictly, but it can result in the next
> allocation rechecking chunks unnecessarily or allocating inodes further
> away from the parent than it needs to. Fix it by swapping the record
> pointer after we update the cached left and right records.
> 
> Fixes: bd169565993b ("xfs: speed up free inode search")
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  fs/xfs/libxfs/xfs_ialloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index ffd5a15d1bb6..abf5beaae907 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -1246,13 +1246,13 @@ xfs_dialloc_ag_inobt(
>  
>  			/* free inodes to the left? */
>  			if (useleft && trec.ir_freecount) {
> -				rec = trec;
>  				xfs_btree_del_cursor(cur, XFS_BTREE_NOERROR);
>  				cur = tcur;
>  
>  				pag->pagl_leftrec = trec.ir_startino;
>  				pag->pagl_rightrec = rec.ir_startino;
>  				pag->pagl_pagino = pagino;
> +				rec = trec;
>  				goto alloc_inode;
>  			}
>  
> -- 
> 2.14.0
> 
> --
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