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Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping
From:   Yang Shi <yang.shi@linux.alibaba.com>
To:     Michal Hocko <mhocko@kernel.org>, Nadav Amit <nadav.amit@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>, ldufour@linux.vnet.ibm.com,
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        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        linux-kernel@vger.kernel.org
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Date:   Fri, 22 Jun 2018 18:01:08 -0700
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Yes, this is true but I guess what Yang Shi meant was that an userspace
>> access racing with munmap is not well defined. You never know whether
>> you get your data, #PTF or SEGV because it depends on timing. The user
>> visible change might be that you lose content and get zero page instead
>> if you hit the race window while we are unmapping which was not possible
>> before. But whouldn't such an access pattern be buggy anyway? You need
>> some form of external synchronization AFAICS.
>>
>> But maybe some userspace depends on "getting right data or get SEGV"
>> semantic. If we have to preserve that then we can come up with a VM_DEAD
>> flag set before we tear it down and force the SEGV on the #PF path.
>> Something similar we already do for MMF_UNSTABLE.
>
> Set VM_DEAD with read mmap_sem held? It should be ok since this is the 
> only place to set this flag for this unique special case.

BTW, it looks the vm flags have used up in 32 bit. If we really need 
VM_DEAD, it should be for both 32-bit and 64-bit.

Any suggestion?

Thanks,
Yang

>
> Yang
>
>