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Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        ldufour@linux.vnet.ibm.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, acme@kernel.org,
        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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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <a52f0585-ebec-d098-2775-f55bde3519a4@linux.alibaba.com>
Date:   Wed, 27 Jun 2018 10:23:39 -0700
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On 6/27/18 12:24 AM, Michal Hocko wrote:
> On Tue 26-06-18 18:03:34, Yang Shi wrote:
>>
>> On 6/26/18 12:43 AM, Peter Zijlstra wrote:
>>> On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote:
>>>> By looking this deeper, we may not be able to cover all the unmapping range
>>>> for VM_DEAD, for example, if the start addr is in the middle of a vma. We
>>>> can't set VM_DEAD to that vma since that would trigger SIGSEGV for still
>>>> mapped area.
>>>>
>>>> splitting can't be done with read mmap_sem held, so maybe just set VM_DEAD
>>>> to non-overlapped vmas. Access to overlapped vmas (first and last) will
>>>> still have undefined behavior.
>>> Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. Acquire
>>> mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for
>>> writing, free everything left, drop mmap_sem.
>>>
>>> ?
>>>
>>> Sure, you acquire the lock 3 times, but both write instances should be
>>> 'short', and I suppose you can do a demote between 1 and 2 if you care.
>> Thanks, Peter. Yes, by looking the code and trying two different approaches,
>> it looks this approach is the most straight-forward one.
> Yes, you just have to be careful about the max vma count limit.

Yes, we should just need copy what do_munmap does as below:

if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
             return -ENOMEM;

If the mas map count limit has been reached, it will return failure 
before zapping mappings.

Thanks,
Yang