On 6/19/18 3:17 PM, Nadav Amit wrote:
> 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() ?

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.

>
> 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,
Yang

>
> Regards,
> Nadav