linux-kernel - Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap

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Message-ID: <94bdfcf0-68ea-404c-a60f-362f677884b6@linux.alibaba.com>
Date:   Fri, 22 Jun 2018 18:01:08 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>, Nadav Amit <nadav.amit@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, ldufour@...ux.vnet.ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping

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
>
>