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

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Date:   Wed, 20 Jun 2018 09:23:17 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, willy@...radead.org,
        ldufour@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, 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



On 6/20/18 12:17 AM, Michal Hocko wrote:
> On Tue 19-06-18 14:13:05, Yang Shi wrote:
>>
>> On 6/19/18 3:02 AM, Peter Zijlstra wrote:
> [...]
>>> Hold up, two things: you having to copy most of do_munmap() didn't seem
>>> to suggest a helper function? And second, since when are we allowed to
>> Yes, they will be extracted into a helper function in the next version.
>>
>> May bad, I don't think it is allowed. We could reform this to:
>>
>> acquire write mmap_sem
>> vma lookup (split vmas)
>> release write mmap_sem
>>
>> acquire read mmap_sem
>> zap pages
>> release read mmap_sem
>>
>> I'm supposed this is safe as what Michal said before.
> I didn't get to read your patches carefully yet but I am wondering why
> do you need to split in the first place. Why cannot you simply unmap the
> range (madvise(DONTNEED)) under the read lock and then take the lock for
> write to finish the rest?

Yes, we can. I just thought splitting vma up-front sounds more straight 
forward. But, I neglected the write mmap_sem issue. Will move the vma 
split into later write mmap_sem in the next version.

Thanks,
Yang