+ autofs-add-comment-about-autofs_mountpoint_changed.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: autofs: add comment about autofs_mountpoint_changed()
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     autofs-add-comment-about-autofs_mountpoint_changed.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/autofs-add-comment-about-autofs_mountpoint_changed.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Ian Kent <raven@themaw.net>
Subject: autofs: add comment about autofs_mountpoint_changed()
Date: Fri, 08 Jul 2022 09:43:18 +0800

The function autofs_mountpoint_changed() is unusual, add a comment about
two cases for which it is needed.

Link: https://lkml.kernel.org/r/165724459804.30914.10974834416046555127.stgit@donald.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/autofs/root.c |   23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

--- a/fs/autofs/root.c~autofs-add-comment-about-autofs_mountpoint_changed
+++ a/fs/autofs/root.c
@@ -291,9 +291,26 @@ static struct dentry *autofs_mountpoint_
 	struct dentry *dentry = path->dentry;
 	struct autofs_sb_info *sbi = autofs_sbi(dentry->d_sb);
 
-	/*
-	 * If this is an indirect mount the dentry could have gone away
-	 * as a result of an expire and a new one created.
+	/* If this is an indirect mount the dentry could have gone away
+	 * and a new one created.
+	 *
+	 * This is unusual and I can't remember the case for which it
+	 * was originally added now. But an example of how this can
+	 * happen is an autofs indirect mount that has the "browse"
+	 * option set and also has the "symlink" option in the autofs
+	 * map entry. In this case the daemon will remove the browse
+	 * directory and create a symlink as the mount leaving the
+	 * struct path stale.
+	 *
+	 * Another not so obvious case is when a mount in an autofs
+	 * indirect mount that uses the "nobrowse" option is being
+	 * expired at the same time as a path walk. If the mount has
+	 * been umounted but the mount point directory seen before
+	 * becoming unhashed (during a lockless path walk) when a stat
+	 * family system call is made the mount won't be re-mounted as
+	 * it should. In this case the mount point that's been removed
+	 * (by the daemon) will be stale and the a new mount point
+	 * dentry created.
 	 */
 	if (autofs_type_indirect(sbi->type) && d_unhashed(dentry)) {
 		struct dentry *parent = dentry->d_parent;
_

Patches currently in -mm which might be from raven@themaw.net are

autofs-use-inode-permission-method-for-write-access.patch
autofs-make-dentry-info-count-consistent.patch
autofs-use-dentry-info-count-instead-of-simple_empty.patch
autofs-add-comment-about-autofs_mountpoint_changed.patch
autofs-remove-unused-ino-field-inode.patch