Commit f27c9298 authored by J. Bruce Fields's avatar J. Bruce Fields Committed by Al Viro
Browse files

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: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent efbf201f

fs/exportfs/expfs.c

+13 −53
Original line number Diff line number Diff line
@@ -69,27 +69,6 @@ find_acceptable_alias(struct dentry *result, 
	return NULL;

}



/*

 * Find root of a disconnected subtree and return a reference to it.

 */

static struct dentry *

find_disconnected_root(struct dentry *dentry)

{

	dget(dentry);

	while (!IS_ROOT(dentry)) {

		struct dentry *parent = dget_parent(dentry);



		if (!(parent->d_flags & DCACHE_DISCONNECTED)) {

			dput(parent);

			break;

		}



		dput(dentry);

		dentry = parent;

	}

	return dentry;

}



static bool dentry_connected(struct dentry *dentry)

{

	dget(dentry);
@@ -225,45 +204,26 @@ out_reconnected: 
static int

reconnect_path(struct vfsmount *mnt, struct dentry *target_dir, char *nbuf)

{

	int err = -ESTALE;

	struct dentry *dentry, *parent;



	while (target_dir->d_flags & DCACHE_DISCONNECTED) {

		struct dentry *dentry = find_disconnected_root(target_dir);

	dentry = dget(target_dir);



	while (dentry->d_flags & DCACHE_DISCONNECTED) {

		BUG_ON(dentry == mnt->mnt_sb->s_root);



		if (!IS_ROOT(dentry)) {

			/* must have found a connected parent - great */

			clear_disconnected(target_dir);

			dput(dentry);

			break;

		} else {

			struct dentry *parent;

			/*

			 * We have hit the top of a disconnected path, try to

			 * find parent and connect.

			 */

		if (IS_ROOT(dentry))

			parent = reconnect_one(mnt, dentry, nbuf);

		else

			parent = dget_parent(dentry);



		if (!parent)

				goto out_reconnected;

			if (IS_ERR(parent)) {

				err = PTR_ERR(parent);

			break;

			}

			dput(parent);

		}

		dput(dentry);

		if (IS_ERR(parent))

			return PTR_ERR(parent);

		dentry = parent;

	}



	if (target_dir->d_flags & DCACHE_DISCONNECTED) {

		/* something went wrong - oh-well */

		if (!err)

			err = -ESTALE;

		return err;

	}



	return 0;

out_reconnected:

	dput(dentry);

	clear_disconnected(target_dir);

	return 0;

}

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