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

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Date:   Fri, 20 Jul 2018 17:09:02 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Sudeep Dutt <sudeep.dutt@...el.com>,
        Ashutosh Dixit <ashutosh.dixit@...el.com>,
        Dimitri Sivanich <sivanich@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Felix Kuehling <felix.kuehling@....com>, kvm@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Christian König <christian.koenig@....com>,
        David Rientjes <rientjes@...gle.com>,
        Leon Romanovsky <leonro@...lanox.com>
Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

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

> From: Michal Hocko <mhocko@...e.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?