Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

[Ian Kent <raven@themaw.net>]

On 18/4/25 16:47, Christian Brauner wrote:
> On Fri, Apr 18, 2025 at 09:20:52AM +0800, Ian Kent wrote:
>> On 18/4/25 09:13, Ian Kent wrote:
>>> On 18/4/25 00:28, Christian Brauner wrote:
>>>> On Thu, Apr 17, 2025 at 05:31:26PM +0200, Sebastian Andrzej Siewior
>>>> wrote:
>>>>> On 2025-04-17 17:28:20 [+0200], Christian Brauner wrote:
>>>>>>>       So if there's some userspace process with a broken
>>>>>>> NFS server and it
>>>>>>>       does umount(MNT_DETACH) it will end up hanging every other
>>>>>>>       umount(MNT_DETACH) on the system because the dealyed_mntput_work
>>>>>>>       workqueue (to my understanding) cannot make progress.
>>>>>> Ok, "to my understanding" has been updated after going back
>>>>>> and reading
>>>>>> the delayed work code. Luckily it's not as bad as I thought it is
>>>>>> because it's queued on system_wq which is multi-threaded so it's at
>>>>>> least not causing everyone with MNT_DETACH to get stuck. I'm still
>>>>>> skeptical how safe this all is.
>>>>> I would (again) throw system_unbound_wq into the game because
>>>>> the former
>>>>> will remain on the CPU on which has been enqueued (if speaking about
>>>>> multi threading).
>>>> Yes, good point.
>>>>
>>>> However, what about using polled grace periods?
>>>>
>>>> A first simple-minded thing to do would be to record the grace period
>>>> after umount_tree() has finished and the check it in namespace_unlock():
>>>>
>>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>>> index d9ca80dcc544..1e7ebcdd1ebc 100644
>>>> --- a/fs/namespace.c
>>>> +++ b/fs/namespace.c
>>>> @@ -77,6 +77,7 @@ static struct hlist_head *mount_hashtable
>>>> __ro_after_init;
>>>>    static struct hlist_head *mountpoint_hashtable __ro_after_init;
>>>>    static struct kmem_cache *mnt_cache __ro_after_init;
>>>>    static DECLARE_RWSEM(namespace_sem);
>>>> +static unsigned long rcu_unmount_seq; /* protected by namespace_sem */
>>>>    static HLIST_HEAD(unmounted);  /* protected by namespace_sem */
>>>>    static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
>>>>    static DEFINE_SEQLOCK(mnt_ns_tree_lock);
>>>> @@ -1794,6 +1795,7 @@ static void namespace_unlock(void)
>>>>           struct hlist_head head;
>>>>           struct hlist_node *p;
>>>>           struct mount *m;
>>>> +       unsigned long unmount_seq = rcu_unmount_seq;
>>>>           LIST_HEAD(list);
>>>>
>>>>           hlist_move_list(&unmounted, &head);
>>>> @@ -1817,7 +1819,7 @@ static void namespace_unlock(void)
>>>>           if (likely(hlist_empty(&head)))
>>>>                   return;
>>>>
>>>> -       synchronize_rcu_expedited();
>>>> +       cond_synchronize_rcu_expedited(unmount_seq);
>>>>
>>>>           hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
>>>>                   hlist_del(&m->mnt_umount);
>>>> @@ -1939,6 +1941,8 @@ static void umount_tree(struct mount *mnt,
>>>> enum umount_tree_flags how)
>>>>                    */
>>>>                   mnt_notify_add(p);
>>>>           }
>>>> +
>>>> +       rcu_unmount_seq = get_state_synchronize_rcu();
>>>>    }
>>>>
>>>>    static void shrink_submounts(struct mount *mnt);
>>>>
>>>>
>>>> I'm not sure how much that would buy us. If it doesn't then it should be
>>>> possible to play with the following possibly strange idea:
>>>>
>>>> diff --git a/fs/mount.h b/fs/mount.h
>>>> index 7aecf2a60472..51b86300dc50 100644
>>>> --- a/fs/mount.h
>>>> +++ b/fs/mount.h
>>>> @@ -61,6 +61,7 @@ struct mount {
>>>>                   struct rb_node mnt_node; /* node in the ns->mounts
>>>> rbtree */
>>>>                   struct rcu_head mnt_rcu;
>>>>                   struct llist_node mnt_llist;
>>>> +               unsigned long mnt_rcu_unmount_seq;
>>>>           };
>>>>    #ifdef CONFIG_SMP
>>>>           struct mnt_pcp __percpu *mnt_pcp;
>>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>>> index d9ca80dcc544..aae9df75beed 100644
>>>> --- a/fs/namespace.c
>>>> +++ b/fs/namespace.c
>>>> @@ -1794,6 +1794,7 @@ static void namespace_unlock(void)
>>>>           struct hlist_head head;
>>>>           struct hlist_node *p;
>>>>           struct mount *m;
>>>> +       bool needs_synchronize_rcu = false;
>>>>           LIST_HEAD(list);
>>>>
>>>>           hlist_move_list(&unmounted, &head);
>>>> @@ -1817,7 +1818,16 @@ static void namespace_unlock(void)
>>>>           if (likely(hlist_empty(&head)))
>>>>                   return;
>>>>
>>>> -       synchronize_rcu_expedited();
>>>> +       hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
>>>> +               if (!poll_state_synchronize_rcu(m->mnt_rcu_unmount_seq))
>>>> +                       continue;
> This has a bug. This needs to be:
>
> 	/* A grace period has already elapsed. */
> 	if (poll_state_synchronize_rcu(m->mnt_rcu_unmount_seq))
> 		continue;
>
> 	/* Oh oh, we have to pay up. */
> 	needs_synchronize_rcu = true;
> 	break;
>
> which I'm pretty sure will eradicate most of the performance gain you've
> seen because fundamentally the two version shouldn't be different (Note,
> I drafted this while on my way out the door. r.
>
> I would test the following version where we pay the cost of the
> smb_mb() from poll_state_synchronize_rcu() exactly one time:
>
> diff --git a/fs/mount.h b/fs/mount.h
> index 7aecf2a60472..51b86300dc50 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -61,6 +61,7 @@ struct mount {
>                  struct rb_node mnt_node; /* node in the ns->mounts rbtree */
>                  struct rcu_head mnt_rcu;
>                  struct llist_node mnt_llist;
> +               unsigned long mnt_rcu_unmount_seq;
>          };
>   #ifdef CONFIG_SMP
>          struct mnt_pcp __percpu *mnt_pcp;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index d9ca80dcc544..dd367c54bc29 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1794,6 +1794,7 @@ static void namespace_unlock(void)
>          struct hlist_head head;
>          struct hlist_node *p;
>          struct mount *m;
> +       unsigned long mnt_rcu_unmount_seq = 0;
>          LIST_HEAD(list);
>
>          hlist_move_list(&unmounted, &head);
> @@ -1817,7 +1818,10 @@ static void namespace_unlock(void)
>          if (likely(hlist_empty(&head)))
>                  return;
>
> -       synchronize_rcu_expedited();
> +       hlist_for_each_entry_safe(m, p, &head, mnt_umount)
> +               mnt_rcu_unmount_seq = max(m->mnt_rcu_unmount_seq, mnt_rcu_unmount_seq);
> +
> +       cond_synchronize_rcu_expedited(mnt_rcu_unmount_seq);
>
>          hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
>                  hlist_del(&m->mnt_umount);
> @@ -1923,8 +1927,10 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
>                          }
>                  }
>                  change_mnt_propagation(p, MS_PRIVATE);
> -               if (disconnect)
> +               if (disconnect) {
> +                       p->mnt_rcu_unmount_seq = get_state_synchronize_rcu();
>                          hlist_add_head(&p->mnt_umount, &unmounted);
> +               }
>
>                  /*
>                   * At this point p->mnt_ns is NULL, notification will be queued
>
> If this doesn't help I had considered recording the rcu sequence number
> during __legitimize_mnt() in the mounts. But we likely can't do that
> because get_state_synchronize_rcu() is expensive because it inserts a
> smb_mb() and that would likely be noticable during path lookup. This
> would also hinge on the notion that the last store of the rcu sequence
> number is guaranteed to be seen when we check them in namespace_unlock().
>
> Another possibly insane idea (haven't fully thought it out but throwing
> it out there to test): allocate a percpu counter for each mount and
> increment it each time we enter __legitimize_mnt() and decrement it when
> we leave __legitimize_mnt(). During umount_tree() check the percpu sum
> for each mount after it's been added to the @unmounted list.

I had been thinking that a completion in the mount with a counter (say

walker_cnt) could be used. Because the mounts are unhashed there won't

be new walks so if/once the count is 0 the walker could call complete()

and wait_for_completion() replaces the rcu sync completely. The catch is

managing walker_cnt correctly could be racy or expensive.


I thought this would not be received to well dew to the additional fields

and it could be a little messy but the suggestion above is a bit similar.


Ian

>
> If we see any mount that has a non-zero count we set a global
> @needs_synchronize_rcu to true and stop counting for the other mounts
> (saving percpu summing cycles). Then call or elide
> synchronize_rcu_expedited() based on the @needs_synchronize_rcu boolean
> in namespace_unlock().
>
> The percpu might make this cheap enough for __legitimize_mnt() to be
> workable (ignoring any other pitfalls I've currently not had time to
> warp my head around).