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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: <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com>
Date:   Tue, 19 Jun 2018 17:31:27 -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>
 <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com>
 <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com>
To:     Yang Shi <yang.shi@linux.alibaba.com>
Sender: linux-kernel-owner@vger.kernel.org
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at 4:08 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:

>=20
>=20
> On 6/19/18 3:17 PM, Nadav Amit wrote:
>> at 4:34 PM, Yang Shi <yang.shi@linux.alibaba.com>
>>  wrote:
>>=20
>>=20
>>> When running some mmap/munmap scalability tests with large memory =
(i.e.
>>>=20
>>>> 300GB), the below hung task issue may happen occasionally.
>>>>=20
>>> 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
>>>=20
>>>=20
>> (snip)
>>=20
>>=20
>>> 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.
>>>=20
>> Does munmap() =3D=3D MADV_DONTNEED + munmap() ?
>=20
> Not exactly the same. So, I basically copied the page zapping used by =
munmap instead of calling MADV_DONTNEED.
>=20
>>=20
>> 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?
>>=20
>=20
> userfaultfd is handled by regular munmap path. So, no change to =
userfaultfd part.

Right. I see it now.

>=20
>>=20
>> 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?
>>=20
>=20
> This should be undefined behavior according to Michal. This has been =
discussed in  https://lwn.net/Articles/753269/.

Thanks for the reference.

Reading the man page I see: "All pages containing a part of the =
indicated
range are unmapped, and subsequent references to these pages will =
generate
SIGSEGV.=E2=80=9D

To me it sounds pretty well-defined, and this implementation does not =
follow
this definition. I would expect the man page to be updated and indicate =
that
the behavior has changed.

Regards,
Nadav=