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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>, willy@infradead.org,
        ldufour@linux.vnet.ibm.com, akpm@linux-foundation.org,
        mingo@redhat.com, acme@kernel.org,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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 <f78924fc-ea81-9ddd-ebb2-28241d5721c8@linux.alibaba.com>
 <20180620071708.GI13685@dhcp22.suse.cz>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <41456a0f-0091-dfdb-952b-9bf08b323ba6@linux.alibaba.com>
Date:   Wed, 20 Jun 2018 09:23:17 -0700
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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