xfs: fix inobt inode allocation search optimization

osandov · djwong · commit c44245b3d543 · 2017-08-11T16:56:33.000-07:00
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: bd16956 ("xfs: speed up free inode search") Signed-off-by: Omar Sandoval <osandov@fb.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git 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;
 			}
 
