exportfs: fix quadratic behavior in filehandle lookup
authorJ. Bruce Fields <bfields@redhat.com>
Fri, 18 Oct 2013 01:34:21 +0000 (21:34 -0400)
committerAl Viro <viro@zeniv.linux.org.uk>
Sat, 9 Nov 2013 05:16:38 +0000 (00:16 -0500)
commitf27c9298fd717e1f7e63e314a7a85a3a7e77139d
treeba217e7745e5c0d7e0279a5d7ddb79cb1a03662b
parentefbf201f7a0be7ffc6532e672fbccb0eed4f5de0
exportfs: fix quadratic behavior in filehandle lookup

Suppose we're given the filehandle for a directory whose closest
ancestor in the dcache is its Nth ancestor.

The main loop in reconnect_path searches for an IS_ROOT ancestor of
target_dir, reconnects that ancestor to its parent, then recommences the
search for an IS_ROOT ancestor from target_dir.

This behavior is quadratic in N.  And there's really no need to restart
the search from target_dir each time: once a directory has been looked
up, it won't become IS_ROOT again.  So instead of starting from
target_dir each time, we can continue where we left off.

This simplifies the code and improves performance on very deep directory
heirachies.  (I can't think of any reason anyone should need heirarchies
a hundred or more deep, but the performance improvement may be valuable
if only to limit damage in case of abuse.)

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
fs/exportfs/expfs.c