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

["Zi Yan" <zi.yan@cs.rutgers.edu>]

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On 6 Nov 2017, at 15:35, Andrea Arcangeli wrote:

> On Mon, Nov 06, 2017 at 10:53:48AM -0500, Zi Yan wrote:
>> Thanks for clarifying it. We both agree that !pmd_present(), which means
>> PMD migration entry, does not get into userfaultfd_must_wait(),
>> then there seems to be no issue with current code yet.
>>
>> However, the if (!pmd_present(_pmd)) in userfaultfd_must_wait() does not
>> match
>> the exact condition. How about the patch below? It can catch pmd
>> migration entries,
>> which are only possible in x86_64 at the moment.
>>
>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>> index 1c713fd5b3e6..dda25444a6ee 100644
>> --- a/fs/userfaultfd.c
>> +++ b/fs/userfaultfd.c
>> @@ -294,9 +294,11 @@ static inline bool userfaultfd_must_wait(struct
>> userfaultfd_ctx *ctx,
>>           * pmd_trans_unstable) of the pmd.
>>           */
>>          _pmd = READ_ONCE(*pmd);
>> -       if (!pmd_present(_pmd))
>> +       if (pmd_none(_pmd))
>>                  goto out;
>>
>> +       VM_BUG_ON(thp_migration_supported() && is_pmd_migration_entry(_pmd));
>> +
>
> As I wrote in prev email I'm not sure about this invariant to be
> correct 100% of the time (plus we'd want a VM_WARN_ON only
> here). Specifically, what does prevent try_to_unmap to run on a THP
> backed mapping with only the mmap_sem for reading?
>

Right. I missed the part that the page table lock is released before
entering handle_userfault(). The pmd_none() can be mapped elsewhere
and migrated, !pmd_present() but not pmd_none() is possible here when
THP migration is enabled.

> I know what prevents to ever reproduce this in practice though (aside
> from the fact the race between the is_swap_pmd() check in the main
> page fault and the above check is small) and it's because compaction
> won't migrate THP and even the numa faults will not use the migration
> entry. So it'd require some more explicit migration numactl while
> userfaults are running to ever see an hang in there.
>
> I think it's a regression since the introduction of THP migration
> around commits 84c3fc4e9c563d8fb91cfdf5948da48fe1af34d3 /
> 616b8371539a6c487404c3b8fb04078016dab4ba /
> 9c670ea37947a82cb6d4df69139f7e46ed71a0ac etc.. before that pmd_none or
> !pmd_present used to be equivalent, not the case any longer. Of course
> pmd_none would have been better before too.
>

Right. Ying’s patch is a fix of the regression.

Fixes: 84c3fc4e9c563 ("mm: thp: check pmd migration entry in common path")

Thanks for pointing all these out.

—
Best Regards,
Yan Zi

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