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Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers
To:     Michal Hocko <mhocko@kernel.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        "David (ChunMing) Zhou" <David1.Zhou@amd.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        David Airlie <airlied@linux.ie>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Mike Marciniszyn <mike.marciniszyn@intel.com>,
        Dennis Dalessandro <dennis.dalessandro@intel.com>,
        Sudeep Dutt <sudeep.dutt@intel.com>,
        Ashutosh Dixit <ashutosh.dixit@intel.com>,
        Dimitri Sivanich <sivanich@sgi.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Felix Kuehling <felix.kuehling@amd.com>, kvm@vger.kernel.org,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, linux-rdma@vger.kernel.org,
        xen-devel@lists.xenproject.org,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        David Rientjes <rientjes@google.com>,
        Leon Romanovsky <leonro@mellanox.com>
References: <20180716115058.5559-1-mhocko@kernel.org>
 <8cbfb09f-0c5a-8d43-1f5e-f3ff7612e289@I-love.SAKURA.ne.jp>
 <20180824113629.GI29735@dhcp22.suse.cz>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <103b1b33-1a1d-27a1-dcf8-5c8ad60056a6@i-love.sakura.ne.jp>
Date:   Fri, 24 Aug 2018 22:02:23 +0900
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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.

Like mn_invl_range_start() involves schedule_delayed_work() which could be
blocked on memory allocation under OOM situation, I worry that (currently
out-of-tree) users of this API are involving work / recursion.
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)".