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

Ian Kent
2025-04-17T10:17:23Z

On 17/4/25 17:01, Christian Brauner wrote:
> On Wed, Apr 16, 2025 at 11:11:51PM +0100, Mark Brown wrote:
>> On Tue, Apr 08, 2025 at 04:58:34PM -0400, Eric Chanudet wrote:
>>> Defer releasing the detached file-system when calling namespace_unlock()
>>> during a lazy umount to return faster.
>>>
>>> When requesting MNT_DETACH, the caller does not expect the file-system
>>> to be shut down upon returning from the syscall. Calling
>>> synchronize_rcu_expedited() has a significant cost on RT kernel that
>>> defaults to rcupdate.rcu_normal_after_boot=1. Queue the detached struct
>>> mount in a separate list and put it on a workqueue to run post RCU
>>> grace-period.
>> For the past couple of days we've been seeing failures in a bunch of LTP
>> filesystem related tests on various arm64 systems. The failures are
>> mostly (I think all) in the form:
>>
>> 20101 10:12:40.378045 tst_test.c:1833: TINFO: === Testing on vfat ===
>> 20102 10:12:40.385091 tst_test.c:1170: TINFO: Formatting /dev/loop0 with vfat opts='' extra opts=''
>> 20103 10:12:40.391032 mkfs.vfat: unable to open /dev/loop0: Device or resource busy
>> 20104 10:12:40.395953 tst_test.c:1170: TBROK: mkfs.vfat failed with exit code 1
>>
>> ie, a failure to stand up the test environment on the loopback device
>> all happening immediately after some other filesystem related test which
>> also used the loop device. A bisect points to commit a6c7a78f1b6b97
>> which is this, which does look rather relevant. LTP is obviously being
>> very much an edge case here.
> Hah, here's something I didn't consider and that I should've caught.
>
> Look, on current mainline no matter if MNT_DETACH/UMOUNT_SYNC or
> non-MNT_DETACH/UMOUNT_SYNC. The mntput() calls after the
> synchronize_rcu_expedited() calls will end up in task_work():
>
> if (likely(!(mnt->mnt.mnt_flags & MNT_INTERNAL))) {
> struct task_struct *task = current;
> if (likely(!(task->flags & PF_KTHREAD))) {
> init_task_work(&mnt->mnt_rcu, __cleanup_mnt);
> if (!task_work_add(task, &mnt->mnt_rcu, TWA_RESUME))
> return;
> }
> if (llist_add(&mnt->mnt_llist, &delayed_mntput_list))
> schedule_delayed_work(&delayed_mntput_work, 1);
> return;
> }
>
> because all of those mntput()s are done from the task's contect.
>
> IOW, if userspace does umount(MNT_DETACH) and the task has returned to
> userspace it is guaranteed that all calls to cleanup_mnt() are done.
>
> With your change that simply isn't true anymore. The call to
> queue_rcu_work() will offload those mntput() to be done from a kthread.
> That in turn means all those mntputs end up on the delayed_mntput_work()
> queue. So the mounts aren't cleaned up by the time the task returns to
> userspace.
>
> And that's likely problematic even for the explicit MNT_DETACH use-case
> because it means EBUSY errors are a lot more likely to be seen by
> concurrent mounters especially for loop devices.
>
> And fwiw, this is exactly what I pointed out in a prior posting to this
> patch series.

And I didn't understand what you said then but this problem is more

understandable to me now.

>
> But we've also worsened that situation by doing the deferred thing for
> any non-UMOUNT_SYNC. That which includes namespace exit. IOW, if the
> last task in a new mount namespace exits it will drop_collected_mounts()
> without UMOUNT_SYNC because we know that they aren't reachable anymore,
> after all the mount namespace is dead.
>
> But now we defer all cleanup to the kthread which means when the task
> returns to userspace there's still mounts to be cleaned up.

Correct me if I'm wrong but the actual problem is that the mechanism used

to wait until there are no processes doing an rcu-walk on mounts in the

discard list is unnecessarily long according to what Eric has seen. So a

different was to know there are no processes doing an rcu-walk for one of

these mounts is needed.

There must be a better way to do this than the current rcu wait method ...

Ian