Date: Wed, 13 Sep 2017 15:57:54 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Sherry Yang <sherryy@android.com>
Cc: linux-kernel@vger.kernel.org, Todd Kjos <tkjos@google.com>,
        Martijn Coenen <maco@google.com>,
        Arve =?ISO-8859-1?Q?Hj=F8nnev=E5g?= <arve@android.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>, Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>, David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        Hoeun Ryu <hoeun.ryu@gmail.com>, Christoph Lameter <cl@linux.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH] mm: Restore mmput_async
Message-Id: <20170913155754.b072c04bbabffab0471a1ee2@linux-foundation.org>
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On Wed, 13 Sep 2017 18:44:11 -0400 Sherry Yang <sherryy@android.com> wrote:

> On Wed, Sep 13, 2017 at 6:09 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Wed, 13 Sep 2017 17:59:27 -0400 Sherry Yang <sherryy@android.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?