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

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Message-ID: <20180717040347.GT3152@mtr-leonro.mtl.com>
Date:   Tue, 17 Jul 2018 07:03:47 +0300
From:   Leon Romanovsky <leonro@...lanox.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        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>
Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

On Mon, Jul 16, 2018 at 04:12:49PM -0700, Andrew Morton wrote:
> 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.

I can't say for everyone, but at least for me (mlx5), it is not rare event.
I'm seeing OOM very often while I'm running my tests in low memory VMs.

Thanks

>
> 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.

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