linux-kernel - Re: [RFC v3 PATCH 5/5] x86: check VM

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Date:   Tue, 3 Jul 2018 08:17:13 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>, willy@...radead.org,
        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 5/5] x86: check VM_DEAD flag in page fault

On Mon 02-07-18 11:10:23, Yang Shi wrote:
> On 7/2/18 10:57 AM, Michal Hocko wrote:
[...]
> > Why would you even care about shared mappings?
> 
> Just thought about we are dealing with VM_DEAD, which means the vma will be
> tore down soon regardless it is shared or non-shared.
> 
> MMF_UNSTABLE doesn't care about !shared case.

Let me clarify some more. MMF_UNSTABLE is there to prevent from
unexpected page faults when the mm is torn down by the oom reaper. And
oom reaper only cares about private mappings because we do not touch
shared ones. Disk based shared mappings should be a non-issue for
VM_DEAD because even if you race and refault a page back then you know
it is the same one you have seen before. Memory backed shared mappings
are a different story because you can get a fresh new page. oom_reaper
doesn't care because it doesn't tear those down. You would have to but
my primary point was that we already have MMF_UNSTABLE so all you need
is to extend it to memory backed shared mappings (shmem and hugetlb).

-- 
Michal Hocko
SUSE Labs