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SubjectRe: [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.



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