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

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Message-Id: <20180716161249.c76240cd487c070fb271d529@linux-foundation.org>
Date:   Mon, 16 Jul 2018 16:12:49 -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.
> 
> Currently we simply back off and mark an oom victim with blockable mmu
> notifiers as done after a short sleep. That can result in selecting a
> new oom victim prematurely because the previous one still hasn't torn
> its memory down yet.
> 
> We can do much better though. Even if mmu notifiers use sleepable locks
> there is no reason to automatically assume those locks are held.
> Moreover majority of notifiers only care about a portion of the address
> space and there is absolutely zero reason to fail when we are unmapping an
> unrelated range. Many notifiers do really block and wait for HW which is
> harder to handle and we have to bail out though.
> 
> This patch handles the low hanging fruid. __mmu_notifier_invalidate_range_start
> gets a blockable flag and callbacks are not allowed to sleep if the
> flag is set to false. This is achieved by using trylock instead of the
> sleepable lock for most callbacks and continue as long as we do not
> block down the call chain.

I assume device driver developers are wondering "what does this mean
for me".  As I understand it, the only time they will see
blockable==false is when their driver is being called in response to an
out-of-memory condition, yes?  So it is a very rare thing.

Any suggestions regarding how the driver developers can test this code
path?  I don't think we presently have a way to fake an oom-killing
event?  Perhaps we should add such a thing, given the problems we're
having with that feature.

> I think we can improve that even further because there is a common
> pattern to do a range lookup first and then do something about that.
> The first part can be done without a sleeping lock in most cases AFAICS.
> 
> The oom_reaper end then simply retries if there is at least one notifier
> which couldn't make any progress in !blockable mode. A retry loop is
> already implemented to wait for the mmap_sem and this is basically the
> same thing.
> 
> ...
>
> +static inline int mmu_notifier_invalidate_range_start_nonblock(struct mm_struct *mm,
> +				  unsigned long start, unsigned long end)
> +{
> +	int ret = 0;
> +	if (mm_has_notifiers(mm))
> +		ret = __mmu_notifier_invalidate_range_start(mm, start, end, false);
> +
> +	return ret;
>  }

nit,

{
	if (mm_has_notifiers(mm))
		return __mmu_notifier_invalidate_range_start(mm, start, end, false);
	return 0;
}

would suffice.


> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3074,7 +3074,7 @@ void exit_mmap(struct mm_struct *mm)
>  		 * reliably test it.
>  		 */
>  		mutex_lock(&oom_lock);
> -		__oom_reap_task_mm(mm);
> +		(void)__oom_reap_task_mm(mm);
>  		mutex_unlock(&oom_lock);

What does this do?

>  		set_bit(MMF_OOM_SKIP, &mm->flags);
> 
> ...
>