Re: [PATCH 1/3] vfs: track count of child mounts

From: Ian Kent <raven@themaw.net>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Howells <dhowells@redhat.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] vfs: track count of child mounts
Date: Tue, 26 Jul 2022 13:11:15 +0800	[thread overview]
Message-ID: <5ee80f66-7e6c-228f-2ee0-a5610dabc887@themaw.net> (raw)
In-Reply-To: <6c072650-aed4-3ea5-0b8b-8e52655a222d@themaw.net>

On 20/7/22 10:17, Ian Kent wrote:
>
> 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.

Just a heads up, I've been able to reliably hang autofs with the

below patch using my submount test (which is actually pretty good

at exposing problems).

No idea what it is yet but I'll look around and keep trying to work

it out, ;)

Ian

>
>
> Ian
>
>>
>> commit e99f1f9cc864103f326a5352e6ce1e377613437f
>> Author: Al Viro <viro@zeniv.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@zeniv.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;