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

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Message-ID: <472ce7f9-be53-c741-6bad-f46ec9dfbb8a@linux.alibaba.com>
Date:   Tue, 3 Jul 2018 09:50:19 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
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 7/2/18 11:17 PM, Michal Hocko wrote:
> 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.

Sorry, this is a typo, it should be "shared".

> 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).

Yes, sure. I think I got your point. Thanks for the elaboration.

Yang

>