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

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Message-ID: <20180703081205.3ue5722pb3ko4g2w@kshutemo-mobl1>
Date:   Tue, 3 Jul 2018 11:12:05 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
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, Jul 02, 2018 at 02:49:28PM +0200, Michal Hocko wrote:
> 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.

Okay, so we add a lot of complexity to accommodate broken userspace that
may or may not exist. Is it right? :)

-- 
 Kirill A. Shutemov