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Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com>
Date:   Tue, 19 Jun 2018 15:17:16 -0700
Cc:     Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        ldufour@linux.vnet.ibm.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.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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References: <1529364856-49589-1-git-send-email-yang.shi@linux.alibaba.com>
 <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com>
To:     Yang Shi <yang.shi@linux.alibaba.com>
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at 4:34 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:

> When running some mmap/munmap scalability tests with large memory (i.e.
>> 300GB), the below hung task issue may happen occasionally.
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>       Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
> ps              D    0 14018      1 0x00000004
> 
(snip)

> 
> Zapping pages is the most time consuming part, according to the
> suggestion from Michal Hock [1], zapping pages can be done with holding
> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write
> mmap_sem to manipulate vmas.

Does munmap() == MADV_DONTNEED + munmap() ?

For example, what happens with userfaultfd in this case? Can you get an
extra #PF, which would be visible to userspace, before the munmap is
finished?

In addition, would it be ok for the user to potentially get a zeroed page in
the time window after the MADV_DONTNEED finished removing a PTE and before
the munmap() is done?

Regards,
Nadav