linux-kernel - [PATCH 1/3] vfs: track count of child mounts

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Date:   Mon, 11 Jul 2022 11:37:40 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH 1/3] vfs: track count of child mounts

While the total reference count of a mount is mostly all that's needed
the reference count corresponding to the mounts only is occassionally
also needed (for example, autofs checking if a tree of mounts can be
expired).

To make this reference count avaialble with minimal changes add a
counter to track the number of child mounts under a given mount. This
count can then be used to calculate the mounts only reference count.

Signed-off-by: Ian Kent <raven@...maw.net>
---
 fs/mount.h     |    1 +
 fs/namespace.c |    8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/fs/mount.h b/fs/mount.h
index 0b6e08cf8afb..3f0f62912463 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -52,6 +52,7 @@ struct mount {
 	int mnt_writers;
 #endif
 	struct list_head mnt_mounts;	/* list of children, anchored here */
+	unsigned int mnt_mounts_cnt;	/* count of children, anchored here */
 	struct list_head mnt_child;	/* and going through their mnt_child */
 	struct list_head mnt_instance;	/* mount instance on sb->s_mounts */
 	const char *mnt_devname;	/* Name of device e.g. /dev/dsk/hda1 */
diff --git a/fs/namespace.c b/fs/namespace.c
index e6a7e769d25d..3c1ee5b5bb69 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -882,6 +882,8 @@ static struct mountpoint *unhash_mnt(struct mount *mnt)
 	struct mountpoint *mp;
 	mnt->mnt_parent = mnt;
 	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
+	if (!list_empty(&mnt->mnt_child))
+		mnt->mnt_parent->mnt_mounts_cnt--;
 	list_del_init(&mnt->mnt_child);
 	hlist_del_init_rcu(&mnt->mnt_hash);
 	hlist_del_init(&mnt->mnt_mp_list);
@@ -918,6 +920,7 @@ static void __attach_mnt(struct mount *mnt, struct mount *parent)
 	hlist_add_head_rcu(&mnt->mnt_hash,
 			   m_hash(&parent->mnt, mnt->mnt_mountpoint));
 	list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+	parent->mnt_mounts_cnt++;
 }
 
 /*
@@ -936,6 +939,8 @@ void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct m
 	struct mountpoint *old_mp = mnt->mnt_mp;
 	struct mount *old_parent = mnt->mnt_parent;
 
+	if (!list_empty(&mnt->mnt_child))
+		mnt->mnt_parent->mnt_mounts_cnt--;
 	list_del_init(&mnt->mnt_child);
 	hlist_del_init(&mnt->mnt_mp_list);
 	hlist_del_init_rcu(&mnt->mnt_hash);
@@ -1562,6 +1567,8 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 
 	/* Hide the mounts from mnt_mounts */
 	list_for_each_entry(p, &tmp_list, mnt_list) {
+		if (!list_empty(&p->mnt_child))
+			p->mnt_parent->mnt_mounts_cnt--;
 		list_del_init(&p->mnt_child);
 	}
 
@@ -1590,6 +1597,7 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 			if (!disconnect) {
 				/* Don't forget about p */
 				list_add_tail(&p->mnt_child, &p->mnt_parent->mnt_mounts);
+				p->mnt_parent->mnt_mounts_cnt++;
 			} else {
 				umount_mnt(p);
 			}