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Date: Wed, 20 Jul 2022 10:17:58 +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: Re: [PATCH 1/3] vfs: track count of child mounts
On 20/7/22 09:50, Al Viro wrote:
> 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.
Right, I'll grab this and run some tests.
Ian
>
> 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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