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Message-Id: <BFD6A249-B1D7-43D5-8D7C-9FAED4A168A1@gmail.com>
Date: Tue, 19 Jun 2018 17:31:27 -0700
From: Nadav Amit <nadav.amit@...il.com>
To: Yang Shi <yang.shi@...ux.alibaba.com>
Cc: Michal Hocko <mhocko@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
ldufour@...ux.vnet.ibm.com,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, acme@...nel.org,
alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
namhyung@...nel.org,
"open list:MEMORY MANAGEMENT" <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
at 4:08 PM, Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
>
> On 6/19/18 3:17 PM, Nadav Amit wrote:
>> at 4:34 PM, Yang Shi <yang.shi@...ux.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() ?
>
> Not exactly the same. So, I basically copied the page zapping used by munmap instead of calling MADV_DONTNEED.
>
>>
>> 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?
>>
>
> userfaultfd is handled by regular munmap path. So, no change to userfaultfd part.
Right. I see it now.
>
>>
>> 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?
>>
>
> 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.”
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
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