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Date:   Mon, 2 Jul 2018 13:48:45 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Yang Shi <yang.shi@linux.alibaba.com>, willy@infradead.org,
        ldufour@linux.vnet.ibm.com, peterz@infradead.org, mingo@redhat.com,
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        linux-kernel@vger.kernel.org
Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for
 large mapping
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On Mon, 2 Jul 2018 16:05:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Fri 29-06-18 20:15:47, Andrew Morton wrote:
> [...]
> > Would one of your earlier designs have addressed all usecases?  I
> > expect the dumb unmap-a-little-bit-at-a-time approach would have?
> 
> It has been already pointed out that this will not work.

I said "one of".  There were others.

> You simply
> cannot drop the mmap_sem during unmap because another thread could
> change the address space under your feet. So you need some form of
> VM_DEAD and handle concurrent and conflicting address space operations.

Unclear that this is a problem.  If a thread does an unmap of a range
of virtual address space, there's no guarantee that upon return some
other thread has not already mapped new stuff into that address range. 
So what's changed?