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

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Date:   Wed, 13 Sep 2017 15:57:54 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sherry Yang <sherryy@...roid.com>
Cc:     linux-kernel@...r.kernel.org, Todd Kjos <tkjos@...gle.com>,
        Martijn Coenen <maco@...gle.com>,
        Arve Hjønnevåg <arve@...roid.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, 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?