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

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

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

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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?

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