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

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Message-ID: <20180702124928.GQ19043@dhcp22.suse.cz>
Date:   Mon, 2 Jul 2018 14:49:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, willy@...radead.org,
        ldufour@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        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 02-07-18 15:33:50, Kirill A. Shutemov wrote:
[...]
> I probably miss the explanation somewhere, but what's wrong with allowing
> other thread to re-populate the VMA?

We have discussed that earlier and it boils down to how is racy access
to munmap supposed to behave. Right now we have either the original
content or SEGV. If we allow to simply madvise_dontneed before real
unmap we could get a new page as well. There might be (quite broken I
would say) user space code that would simply corrupt data silently that
way.
-- 
Michal Hocko
SUSE Labs