Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

Andrew Morton
2018-07-21T00:09:02Z

On Mon, 16 Jul 2018 13:50:58 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> From: Michal Hocko <mhocko@suse.com>
>
> There are several blockable mmu notifiers which might sleep in
> mmu_notifier_invalidate_range_start and that is a problem for the
> oom_reaper because it needs to guarantee a forward progress so it cannot
> depend on any sleepable locks.
>
> ...
>
> @@ -571,7 +565,12 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>
> trace_start_task_reaping(tsk->pid);
>
> - __oom_reap_task_mm(mm);
> + /* failed to reap part of the address space. Try again later */
> + if (!__oom_reap_task_mm(mm)) {
> + up_read(&mm->mmap_sem);
> + ret = false;
> + goto unlock_oom;
> + }

This function is starting to look a bit screwy.

: static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
: {
: if (!down_read_trylock(&mm->mmap_sem)) {
: trace_skip_task_reaping(tsk->pid);
: return false;
: }
:
: /*
: * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
: * work on the mm anymore. The check for MMF_OOM_SKIP must run
: * under mmap_sem for reading because it serializes against the
: * down_write();up_write() cycle in exit_mmap().
: */
: if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
: up_read(&mm->mmap_sem);
: trace_skip_task_reaping(tsk->pid);
: return true;
: }
:
: trace_start_task_reaping(tsk->pid);
:
: /* failed to reap part of the address space. Try again later */
: if (!__oom_reap_task_mm(mm)) {
: up_read(&mm->mmap_sem);
: return true;
: }
:
: pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
: task_pid_nr(tsk), tsk->comm,
: K(get_mm_counter(mm, MM_ANONPAGES)),
: K(get_mm_counter(mm, MM_FILEPAGES)),
: K(get_mm_counter(mm, MM_SHMEMPAGES)));
: up_read(&mm->mmap_sem);
:
: trace_finish_task_reaping(tsk->pid);
: return true;
: }

- Undocumented return value.

- comment "failed to reap part..." is misleading - sounds like it's
referring to something which happened in the past, is in fact
referring to something which might happen in the future.

- fails to call trace_finish_task_reaping() in one case

- code duplication.

I'm thinking it wants to be something like this?

: /*
: * Return true if we successfully acquired (then released) mmap_sem
: */
: static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
: {
: if (!down_read_trylock(&mm->mmap_sem)) {
: trace_skip_task_reaping(tsk->pid);
: return false;
: }
:
: /*
: * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
: * work on the mm anymore. The check for MMF_OOM_SKIP must run
: * under mmap_sem for reading because it serializes against the
: * down_write();up_write() cycle in exit_mmap().
: */
: if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
: trace_skip_task_reaping(tsk->pid);
: goto out;
: }
:
: trace_start_task_reaping(tsk->pid);
:
: if (!__oom_reap_task_mm(mm)) {
: /* Failed to reap part of the address space. Try again later */
: goto finish;
: }
:
: pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
: task_pid_nr(tsk), tsk->comm,
: K(get_mm_counter(mm, MM_ANONPAGES)),
: K(get_mm_counter(mm, MM_FILEPAGES)),
: K(get_mm_counter(mm, MM_SHMEMPAGES)));
: finish:
: trace_finish_task_reaping(tsk->pid);
: out:
: up_read(&mm->mmap_sem);
: return true;
: }

- Increases mmap_sem hold time a little by moving
trace_finish_task_reaping() inside the locked region. So sue me ;)

- Sharing the finish: path means that the trace event won't
distinguish between the two sources of finishing.

Please take a look?