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

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Message-ID: <20180824133207.GR29735@dhcp22.suse.cz>
Date:   Fri, 24 Aug 2018 15:32:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Jérôme Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        "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>,
        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 Fri 24-08-18 22:02:23, Tetsuo Handa wrote:
> On 2018/08/24 20:36, Michal Hocko wrote:
> >> That is, this API seems to be currently used by only out-of-tree users. Since
> >> we can't check that nobody has memory allocation dependency, I think that
> >> hmm_invalidate_range_start() should return -EAGAIN if blockable == false for now.
> > 
> > The code expects that the invalidate_range_end doesn't block if
> > invalidate_range_start hasn't blocked. That is the reason why the end
> > callback doesn't have blockable parameter. If this doesn't hold then the
> > whole scheme is just fragile because those two calls should pair.
> > 
> That is
> 
>   More worrisome part in that patch is that I don't know whether using
>   trylock if blockable == false at entry is really sufficient.
> 
> . Since those two calls should pair, I think that we need to determine whether
> we need to return -EAGAIN at start call by evaluating both calls.

Yes, and I believe I have done that audit. Module my misunderstanding of
the code.

> Like mn_invl_range_start() involves schedule_delayed_work() which could be
> blocked on memory allocation under OOM situation,

It doesn't because that code path is not invoked for the !blockable
case.

> I worry that (currently
> out-of-tree) users of this API are involving work / recursion.

I do not give a slightest about out-of-tree modules. They will have to
accomodate to the new API. I have no problems to extend the
documentation and be explicit about this expectation.
diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index 133ba78820ee..698e371aafe3 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -153,7 +153,9 @@ struct mmu_notifier_ops {
 	 *
 	 * If blockable argument is set to false then the callback cannot
 	 * sleep and has to return with -EAGAIN. 0 should be returned
-	 * otherwise.
+	 * otherwise. Please note that if invalidate_range_start approves
+	 * a non-blocking behavior then the same applies to
+	 * invalidate_range_end.
 	 *
 	 */
 	int (*invalidate_range_start)(struct mmu_notifier *mn,


> And hmm_release() says that
> 
> 	/*
> 	 * Drop mirrors_sem so callback can wait on any pending
> 	 * work that might itself trigger mmu_notifier callback
> 	 * and thus would deadlock with us.
> 	 */
> 
> and keeps "all operations protected by hmm->mirrors_sem held for write are
> atomic". This suggests that "some operations protected by hmm->mirrors_sem held
> for read will sleep (and in the worst case involves memory allocation
> dependency)".

Yes and so what? The clear expectation is that neither of the range
notifiers do not sleep in !blocking mode. I really fail to see what you
are trying to say.

-- 
Michal Hocko
SUSE Labs