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

Ian Kent
2025-04-10T01:17:32Z

On 9/4/25 18:37, Christian Brauner 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.
>>
>> w/o patch, 6.15-rc1 PREEMPT_RT:
>> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>> 0.02455 +- 0.00107 seconds time elapsed ( +- 4.36% )
>> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>> 0.02555 +- 0.00114 seconds time elapsed ( +- 4.46% )
>>
>> w/ patch, 6.15-rc1 PREEMPT_RT:
>> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
>> 0.026311 +- 0.000869 seconds time elapsed ( +- 3.30% )
>> perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount -l mnt
>> 0.003194 +- 0.000160 seconds time elapsed ( +- 5.01% )
>>
>> Signed-off-by: Alexander Larsson <alexl@redhat.com>
>> Signed-off-by: Lucas Karpinski <lkarpins@redhat.com>
>> Signed-off-by: Eric Chanudet <echanude@redhat.com>
>> ---
>>
>> Attempt to re-spin this series based on the feedback received in v3 that
>> pointed out the need to wait the grace-period in namespace_unlock()
>> before calling the deferred mntput().
> I still hate this with a passion because it adds another special-sauce
> path into the unlock path. I've folded the following diff into it so it
> at least doesn't start passing that pointless boolean and doesn't
> introduce __namespace_unlock(). Just use a global variable and pick the
> value off of it just as we do with the lists. Testing this now:

Yeah, it's painful that's for sure.

I also agree with you about the parameter, changing the call signature

always rubbed me the wrong way but I didn't push back on it mostly because

we needed to find a way to do it sensibly and it sounds like that's still

the case.

AFAICT what's needed is a way to synchronize umount with the lockless path

walk. Now umount detaches the mounts concerned, calls the rcu synchronize

(essentially sleeps) to ensure that any lockless path walks see the umount

before completing. But that rcu sync. is, as we can see, really wasteful so

we do need to find a viable way to synchronize this.

Strictly speaking the synchronization problem exists for normal and detached

umounts but if we can find a sound solution for detached mounts perhaps
we can

extend later (but now that seems like a stretch) ...

I'm not sure why, perhaps it's just me, I don't know, but with this we don't

seem to be working well together to find a solution, I hope we can
change that

this time around.

I was thinking of using a completion for this synchronization but even that

would be messy because of possible multiple processes doing walks at the
time

which doesn't lend cleanly to using a completion.

Do you have any ideas on how this could be done yourself?

Ian

>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index e5b0b920dd97..25599428706c 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -82,8 +82,9 @@ 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 HLIST_HEAD(unmounted); /* protected by namespace_sem */
> -static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
> +static bool unmounted_lazily; /* 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);
>
> #ifdef CONFIG_FSNOTIFY
> @@ -1807,17 +1808,18 @@ static void free_mounts(struct hlist_head *mount_list)
>
> static void defer_free_mounts(struct work_struct *work)
> {
> - struct deferred_free_mounts *d = container_of(
> - to_rcu_work(work), struct deferred_free_mounts, rwork);
> + struct deferred_free_mounts *d;
>
> + d = container_of(to_rcu_work(work), struct deferred_free_mounts, rwork);
> free_mounts(&d->release_list);
> kfree(d);
> }
>
> -static void __namespace_unlock(bool lazy)
> +static void namespace_unlock(void)
> {
> HLIST_HEAD(head);
> LIST_HEAD(list);
> + bool defer = unmounted_lazily;
>
> hlist_move_list(&unmounted, &head);
> list_splice_init(&ex_mountpoints, &list);
> @@ -1840,29 +1842,21 @@ static void __namespace_unlock(bool lazy)
> if (likely(hlist_empty(&head)))
> return;
>
> - if (lazy) {
> - struct deferred_free_mounts *d =
> - kmalloc(sizeof(*d), GFP_KERNEL);
> + if (defer) {
> + struct deferred_free_mounts *d;
>
> - if (unlikely(!d))
> - goto out;
> -
> - hlist_move_list(&head, &d->release_list);
> - INIT_RCU_WORK(&d->rwork, defer_free_mounts);
> - queue_rcu_work(system_wq, &d->rwork);
> - return;
> + d = kmalloc(sizeof(struct deferred_free_mounts), GFP_KERNEL);
> + if (d) {
> + hlist_move_list(&head, &d->release_list);
> + INIT_RCU_WORK(&d->rwork, defer_free_mounts);
> + queue_rcu_work(system_wq, &d->rwork);
> + return;
> + }
> }
> -
> -out:
> synchronize_rcu_expedited();
> free_mounts(&head);
> }
>
> -static inline void namespace_unlock(void)
> -{
> - __namespace_unlock(false);
> -}
> -
> static inline void namespace_lock(void)
> {
> down_write(&namespace_sem);
> @@ -2094,7 +2088,7 @@ static int do_umount(struct mount *mnt, int flags)
> }
> out:
> unlock_mount_hash();
> - __namespace_unlock(flags & MNT_DETACH);
> + namespace_unlock();
> return retval;
> }
>
>
>> v4:
>> - Use queue_rcu_work() to defer free_mounts() for lazy umounts
>> - Drop lazy_unlock global and refactor using a helper
>> v3: https://lore.kernel.org/all/20240626201129.272750-2-lkarpins@redhat.com/
>> - Removed unneeded code for lazy umount case.
>> - Don't block within interrupt context.
>> v2: https://lore.kernel.org/all/20240426195429.28547-1-lkarpins@redhat.com/
>> - Only defer releasing umount'ed filesystems for lazy umounts
>> v1: https://lore.kernel.org/all/20230119205521.497401-1-echanude@redhat.com/
>>
>> fs/namespace.c | 52 +++++++++++++++++++++++++++++++++++++++++++-------
>> 1 file changed, 45 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 14935a0500a2..e5b0b920dd97 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -45,6 +45,11 @@ static unsigned int m_hash_shift __ro_after_init;
>> static unsigned int mp_hash_mask __ro_after_init;
>> static unsigned int mp_hash_shift __ro_after_init;
>>
>> +struct deferred_free_mounts {
>> + struct rcu_work rwork;
>> + struct hlist_head release_list;
>> +};
>> +
>> static __initdata unsigned long mhash_entries;
>> static int __init set_mhash_entries(char *str)
>> {
>> @@ -1789,11 +1794,29 @@ static bool need_notify_mnt_list(void)
>> }
>> #endif
>>
>> -static void namespace_unlock(void)
>> +static void free_mounts(struct hlist_head *mount_list)
>> {
>> - struct hlist_head head;
>> struct hlist_node *p;
>> struct mount *m;
>> +
>> + hlist_for_each_entry_safe(m, p, mount_list, mnt_umount) {
>> + hlist_del(&m->mnt_umount);
>> + mntput(&m->mnt);
>> + }
>> +}
>> +
>> +static void defer_free_mounts(struct work_struct *work)
>> +{
>> + struct deferred_free_mounts *d = container_of(
>> + to_rcu_work(work), struct deferred_free_mounts, rwork);
>> +
>> + free_mounts(&d->release_list);
>> + kfree(d);
>> +}
>> +
>> +static void __namespace_unlock(bool lazy)
>> +{
>> + HLIST_HEAD(head);
>> LIST_HEAD(list);
>>
>> hlist_move_list(&unmounted, &head);
>> @@ -1817,12 +1840,27 @@ static void namespace_unlock(void)
>> if (likely(hlist_empty(&head)))
>> return;
>>
>> - synchronize_rcu_expedited();
>> + if (lazy) {
>> + struct deferred_free_mounts *d =
>> + kmalloc(sizeof(*d), GFP_KERNEL);
>>
>> - hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
>> - hlist_del(&m->mnt_umount);
>> - mntput(&m->mnt);
>> + if (unlikely(!d))
>> + goto out;
>> +
>> + hlist_move_list(&head, &d->release_list);
>> + INIT_RCU_WORK(&d->rwork, defer_free_mounts);
>> + queue_rcu_work(system_wq, &d->rwork);
>> + return;
>> }
>> +
>> +out:
>> + synchronize_rcu_expedited();
>> + free_mounts(&head);
>> +}
>> +
>> +static inline void namespace_unlock(void)
>> +{
>> + __namespace_unlock(false);
>> }
>>
>> static inline void namespace_lock(void)
>> @@ -2056,7 +2094,7 @@ static int do_umount(struct mount *mnt, int flags)
>> }
>> out:
>> unlock_mount_hash();
>> - namespace_unlock();
>> + __namespace_unlock(flags & MNT_DETACH);
>> return retval;
>> }
>>
>> --
>> 2.49.0
>>