linux-kernel - Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap

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Message-Id: <20180702134845.c4f536dead5374b443e24270@linux-foundation.org>
Date:   Mon, 2 Jul 2018 13:48:45 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, willy@...radead.org,
        ldufour@...ux.vnet.ibm.com, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, tglx@...utronix.de,
        hpa@...or.com, linux-mm@...ck.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for
 large mapping

On Mon, 2 Jul 2018 16:05:02 +0200 Michal Hocko <mhocko@...nel.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?