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Date: Wed, 20 Jul 2022 02:50:10 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Ian Kent <raven@...maw.net>
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: Re: [PATCH 1/3] vfs: track count of child mounts
On Mon, Jul 11, 2022 at 11:37:40AM +0800, Ian Kent wrote:
> 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.
No. This is a wrong approach - instead of keeping track of number of
children, we should just stop having them contribute to refcount of
the parent. Here's what I've got in my local tree; life gets simpler
that way.
commit e99f1f9cc864103f326a5352e6ce1e377613437f
Author: Al Viro <viro@...iv.linux.org.uk>
Date: Sat Jul 9 14:45:39 2022 -0400
namespace: don't keep ->mnt_parent pinned
makes refcounting more consistent
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
diff --git a/fs/namespace.c b/fs/namespace.c
index 68789f896f08..53c29110a0cd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -906,7 +906,6 @@ void mnt_set_mountpoint(struct mount *mnt,
struct mount *child_mnt)
{
mp->m_count++;
- mnt_add_count(mnt, 1); /* essentially, that's mntget */
child_mnt->mnt_mountpoint = mp->m_dentry;
child_mnt->mnt_parent = mnt;
child_mnt->mnt_mp = mp;
@@ -1429,22 +1428,18 @@ void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor)
int may_umount_tree(struct vfsmount *m)
{
struct mount *mnt = real_mount(m);
- int actual_refs = 0;
- int minimum_refs = 0;
- struct mount *p;
BUG_ON(!m);
/* write lock needed for mnt_get_count */
lock_mount_hash();
- for (p = mnt; p; p = next_mnt(p, mnt)) {
- actual_refs += mnt_get_count(p);
- minimum_refs += 2;
+ for (struct mount *p = mnt; p; p = next_mnt(p, mnt)) {
+ int allowed = p == mnt ? 2 : 1;
+ if (mnt_get_count(p) > allowed) {
+ unlock_mount_hash();
+ return 0;
+ }
}
unlock_mount_hash();
-
- if (actual_refs > minimum_refs)
- return 0;
-
return 1;
}
@@ -1586,7 +1581,6 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
disconnect = disconnect_mount(p, how);
if (mnt_has_parent(p)) {
- mnt_add_count(p->mnt_parent, -1);
if (!disconnect) {
/* Don't forget about p */
list_add_tail(&p->mnt_child, &p->mnt_parent->mnt_mounts);
@@ -2892,12 +2886,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
put_mountpoint(old_mp);
out:
unlock_mount(mp);
- if (!err) {
- if (attached)
- mntput_no_expire(parent);
- else
- free_mnt_ns(ns);
- }
+ if (!err && !attached)
+ free_mnt_ns(ns);
return err;
}
@@ -3869,7 +3859,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
const char __user *, put_old)
{
struct path new, old, root;
- struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent, *ex_parent;
+ struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent;
struct mountpoint *old_mp, *root_mp;
int error;
@@ -3900,10 +3890,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
new_mnt = real_mount(new.mnt);
root_mnt = real_mount(root.mnt);
old_mnt = real_mount(old.mnt);
- ex_parent = new_mnt->mnt_parent;
root_parent = root_mnt->mnt_parent;
if (IS_MNT_SHARED(old_mnt) ||
- IS_MNT_SHARED(ex_parent) ||
+ IS_MNT_SHARED(new_mnt->mnt_parent) ||
IS_MNT_SHARED(root_parent))
goto out4;
if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
@@ -3942,7 +3931,6 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
attach_mnt(root_mnt, old_mnt, old_mp);
/* mount new_root on / */
attach_mnt(new_mnt, root_parent, root_mp);
- mnt_add_count(root_parent, -1);
touch_mnt_namespace(current->nsproxy->mnt_ns);
/* A moved mount should not expire automatically */
list_del_init(&new_mnt->mnt_expire);
@@ -3952,8 +3940,6 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
error = 0;
out4:
unlock_mount(old_mp);
- if (!error)
- mntput_no_expire(ex_parent);
out3:
path_put(&root);
out2:
diff --git a/fs/pnode.c b/fs/pnode.c
index 1106137c747a..e2c8a4b18857 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -368,7 +368,7 @@ static inline int do_refcount_check(struct mount *mnt, int count)
*/
int propagate_mount_busy(struct mount *mnt, int refcnt)
{
- struct mount *m, *child, *topper;
+ struct mount *m, *child;
struct mount *parent = mnt->mnt_parent;
if (mnt == parent)
@@ -384,7 +384,6 @@ int propagate_mount_busy(struct mount *mnt, int refcnt)
for (m = propagation_next(parent, parent); m;
m = propagation_next(m, parent)) {
- int count = 1;
child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
if (!child)
continue;
@@ -392,13 +391,10 @@ int propagate_mount_busy(struct mount *mnt, int refcnt)
/* Is there exactly one mount on the child that covers
* it completely whose reference should be ignored?
*/
- topper = find_topper(child);
- if (topper)
- count += 1;
- else if (!list_empty(&child->mnt_mounts))
+ if (!find_topper(child) && !list_empty(&child->mnt_mounts))
continue;
- if (do_refcount_check(child, count))
+ if (do_refcount_check(child, 1))
return 1;
}
return 0;
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