linux-kernel - Re: [PATCH] mm: Restore mmput

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Message-ID: <CAMP5Xgegepv1nXKgj-mVDFWi-W0G0JBN0KdpHo4sKLv08HX6=A@mail.gmail.com>
Date:   Wed, 13 Sep 2017 17:08:47 -0700
From:   Arve Hjønnevåg <arve@...roid.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Sherry Yang <sherryy@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Todd Kjos <tkjos@...gle.com>, Martijn Coenen <maco@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Hoeun Ryu <hoeun.ryu@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] mm: Restore mmput_async

On Wed, Sep 13, 2017 at 3:57 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Wed, 13 Sep 2017 18:44:11 -0400 Sherry Yang <sherryy@...roid.com> wrote:
>
>> On Wed, Sep 13, 2017 at 6:09 PM, Andrew Morton
>> <akpm@...ux-foundation.org> wrote:
>> > On Wed, 13 Sep 2017 17:59:27 -0400 Sherry Yang <sherryy@...roid.com> wrote:
>> >
>> >> Restore asynchronous mmput, allowing mmput_async to be called
>> >> from an atomic context in Android binder shrinker callback.
>> >>
>> >> mmput_async was initially introduced in ec8d7c14e
>> >> ("mm, oom_reaper: do not mmput synchronously from the
>> >> oom reaper context"), and was removed in 212925802
>> >> ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
>> >
>> > Presumably there's a patch somewhere which adds a call to mmput_async()
>> > into drivers/android/binder.c?  Where is that patch?
>>
>> The patch that uses mmput_async() is
>> https://lkml.org/lkml/2017/9/8/785. Gmail doesn't seem to respect
>> in-reply-to.
>
> (Top-posting repaired.  Please don't!)
>
> Is it necessary for binder_alloc_free_page() to take a ref on the mm?
> As long as alloc->tsk doesn't exit during binder_alloc_free_page()'s
> execution, that task's reference on the mm should be sufficient to keep
> the mm alive?
>

alloc->tsk can exit during binder_alloc_free_page. We don't hold a
reference to the task's mm struct while we don't actively use it, as
this would prevent the driver from getting closed when a process dies.

-- 
Arve Hjønnevåg