Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration

[huang ying]

On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@xxxxxxxxxxxxxx> wrote:
> On 3 Nov 2017, at 3:52, Huang, Ying wrote:
>
>> From: Huang Ying <ying.huang@xxxxxxxxx>
>>
>> If THP migration is enabled, the following situation is possible,
>>
>> - A THP is mapped at source address
>> - Migration is started to move the THP to another node
>> - Page fault occurs
>> - The PMD (migration entry) is copied to the destination address in mremap
>>
>
> You mean the page fault path follows the source address and sees pmd_none() now
> because mremap() clears it and remaps the page with dest address.
> Otherwise, it seems not possible to get into handle_userfault(), since it is called in
> pmd_none() branch inside do_huge_pmd_anonymous_page().
>
>
>> That is, it is possible for handle_userfault() encounter a PMD entry
>> which has been handled but !pmd_present().  In the current
>> implementation, we will wait for such PMD entries, which may cause
>> unnecessary waiting, and potential soft lockup.
>
> handle_userfault() should only see pmd_none() in the situation you describe,
> whereas !pmd_present() (migration entry case) should lead to
> pmd_migration_entry_wait().

Yes.  This is my understanding of the source code too.  And I
described it in the original patch description too.  I just want to
make sure whether it is possible that !pmd_none() and !pmd_present()
for a PMD in userfaultfd_must_wait().  And, whether it is possible for
us to implement PMD mapping copying in UFFDIO_COPY in the future?

Best Regards,
Huang, Ying

> Am I missing anything here?
>
>
> --
> Best Regards
> Yan Zi