Re: [PATCH v2] mm: optimize the redundant loop of mm_update_next_owner()

Michal Hocko
2024-06-27T07:44:53Z

On Thu 20-06-24 23:27:45, alexjlzheng@gmail.com wrote:
> From: Jinliang Zheng <alexjlzheng@tencent.com>
>
> When mm_update_next_owner() is racing with swapoff (try_to_unuse()) or /proc or
> ptrace or page migration (get_task_mm()), it is impossible to find an
> appropriate task_struct in the loop whose mm_struct is the same as the target
> mm_struct.
>
> If the above race condition is combined with the stress-ng-zombie and
> stress-ng-dup tests, such a long loop can easily cause a Hard Lockup in
> write_lock_irq() for tasklist_lock.
>
> Recognize this situation in advance and exit early.
>
> Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>

Even if this is not really a full fix it is a useful stop gap to catch
at least some cases.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changelog:
>
> V2: Fix mm_update_owner_next() to mm_update_next_owner() in comment
> ---
> kernel/exit.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index f95a2c1338a8..81fcee45d630 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -484,6 +484,8 @@ void mm_update_next_owner(struct mm_struct *mm)
> * Search through everything else, we should not get here often.
> */
> for_each_process(g) {
> + if (atomic_read(&mm->mm_users) <= 1)
> + break;
> if (g->flags & PF_KTHREAD)
> continue;
> for_each_thread(g, c) {
> --
> 2.39.3
>

--
Michal Hocko
SUSE Labs