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

Ian Kent
2025-04-10T14:05:04Z

On 10/4/25 00:04, Christian Brauner wrote:
> On Wed, Apr 09, 2025 at 04:25:10PM +0200, Sebastian Andrzej Siewior wrote:
>> On 2025-04-09 16:02:29 [+0200], Mateusz Guzik wrote:
>>> On Wed, Apr 09, 2025 at 03:14:44PM +0200, Sebastian Andrzej Siewior wrote:
>>>> One question: Do we need this lazy/ MNT_DETACH case? Couldn't we handle
>>>> them all via queue_rcu_work()?
>>>> If so, couldn't we have make deferred_free_mounts global and have two
>>>> release_list, say release_list and release_list_next_gp? The first one
>>>> will be used if queue_rcu_work() returns true, otherwise the second.
>>>> Then once defer_free_mounts() is done and release_list_next_gp not
>>>> empty, it would move release_list_next_gp -> release_list and invoke
>>>> queue_rcu_work().
>>>> This would avoid the kmalloc, synchronize_rcu_expedited() and the
>>>> special-sauce.
>>>>
>>> To my understanding it was preferred for non-lazy unmount consumers to
>>> wait until the mntput before returning.
>>>
>>> That aside if I understood your approach it would de facto serialize all
>>> of these?
>>>
>>> As in with the posted patches you can have different worker threads
>>> progress in parallel as they all get a private list to iterate.
>>>
>>> With your proposal only one can do any work.
>>>
>>> One has to assume with sufficient mount/unmount traffic this can
>>> eventually get into trouble.
>> Right, it would serialize them within the same worker thread. With one
>> worker for each put you would schedule multiple worker from the RCU
>> callback. Given the system_wq you will schedule them all on the CPU
>> which invokes the RCU callback. This kind of serializes it, too.
>>
>> The mntput() callback uses spinlock_t for locking and then it frees
>> resources. It does not look like it waits for something nor takes ages.
>> So it might not be needed to split each put into its own worker on a
>> different CPU… One busy bee might be enough ;)
> Unmounting can trigger very large number of mounts to be unmounted. If
> you're on a container heavy system or services that all propagate to
> each other in different mount namespaces mount propagation will generate
> a ton of umounts. So this cannot be underestimated.
>
> If a mount tree is wasted without MNT_DETACH it will pass UMOUNT_SYNC to
> umount_tree(). That'll cause MNT_SYNC_UMOUNT to be raised on all mounts
> during the unmount.
>
> If a concurrent path lookup calls legitimize_mnt() on such a mount and
> sees that MNT_SYNC_UMOUNT is set it will discount as it know that the
> concurrent unmounter hold the last reference and it __legitimize_mnt()
> can thus simply drop the reference count. The final mntput() will be
> done by the umounter.

In umount_tree() it looks like the unmounted mount remains hashed (ie.

disconnect_mount() returns false) so can't it still race with an rcu-walk

regardless of the sybcronsize_rcu().

Surely I'm missing something ...

Ian

>
> The synchronize_rcu() call in namespace_unlock() takes care that the
> last mntput() doesn't happen until path walking has dropped out of RCU
> mode.
>
> Without it it's possible that a non-MNT_DETACH umounter gets a spurious
> EBUSY error because a concurrent lazy path walk will suddenly put the
> last reference via mntput().
>
> I'm unclear how that's handled in whatever it is you're proposing.
>