Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping

[Nadav Amit]

at 12:18 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> On Tue 19-06-18 17:31:27, Nadav Amit wrote:
>> at 4:08 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>> 
>>> On 6/19/18 3:17 PM, Nadav Amit wrote:
>>>> at 4:34 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>>>> 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.â
> 
> Yes, this is true but I guess what Yang Shi meant was that an userspace
> access racing with munmap is not well defined. You never know whether
> you get your data, #PTF or SEGV because it depends on timing. The user
> visible change might be that you lose content and get zero page instead
> if you hit the race window while we are unmapping which was not possible
> before. But whouldn't such an access pattern be buggy anyway? You need
> some form of external synchronization AFAICS.

It seems to follow the specifications, so it is not clearly buggy IMHO. I
donât know of such a use-case, but if somebody does so - the proposed change
might even cause a security vulnerability.

> But maybe some userspace depends on "getting right data or get SEGV"
> semantic. If we have to preserve that then we can come up with a VM_DEAD
> flag set before we tear it down and force the SEGV on the #PF path.
> Something similar we already do for MMF_UNSTABLE.

That seems reasonable.

Regards,
Nadav

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