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Subject: Re: [RFC v3 PATCH 5/5] x86: check VM_DEAD flag in page fault
To:     Michal Hocko <mhocko@kernel.org>,
        Laurent Dufour <ldufour@linux.vnet.ibm.com>
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <6fd4eb3d-ef66-7a37-4adb-05c22ac51d95@linux.alibaba.com>
Date:   Mon, 2 Jul 2018 10:24:27 -0700
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On 7/2/18 6:37 AM, Michal Hocko wrote:
> On Mon 02-07-18 15:33:11, Laurent Dufour wrote:
>>
>> On 02/07/2018 14:45, Michal Hocko wrote:
>>> On Mon 02-07-18 14:26:09, Laurent Dufour wrote:
>>>> On 02/07/2018 14:15, Michal Hocko wrote:
> [...]
>>>>> We already do have a model for that. Have a look at MMF_UNSTABLE.
>>>> MMF_UNSTABLE is a mm's flag, here this is a VMA's flag which is checked.
>>> Yeah, and we have the VMA ready for all places where we do check the
>>> flag. check_stable_address_space can be made to get vma rather than mm.
>> Yeah, this would have been more efficient to check that flag at the beginning
>> of the page fault handler rather than the end, but this way it will be easier
>> to handle the speculative page fault too ;)
> The thing is that it doesn't really need to be called earlier. You are
> not risking data corruption on file backed mappings.

OK, I just think it could save a few cycles to check the flag earlier.

If nobody think it is necessary, we definitely could re-use 
check_stable_address_space(), just return VM_FAULT_SIGSEGV for VM_DEAD 
vma, and check for both shared and non-shared.

Thanks,
Yang

>