Re: [PATCH 2/2] mm/mempolicy: Skip walking HUGETLB vma if MPOL_MF_STRICT is specified alone

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-01-16 at 16:07 HORIGUCHI NAOYA(堀口　直也) wrote:
>Hi everyone,
>
>Thank you all for finding and digging the issue.
>
>> Summary
>> =======
>> It 'looks' like the statement "MPOL_MF_STRICT is ignored on huge page
>> mappings." is left over from the original mbind implementation.  When
>> the huge page migration support was added, I can not be sure if ignoring
>> MPOL_MF_STRICT for huge pages during the verify/isolation phase was
>> intentional.  It seems like it was as the return value from
>> isolate_huge_page() is ignored.
>
>This summary is totally correct.  I've simply missed considering MPOL_MF_STRICT
>flag when implementing hugetlb migration.  As you pointed out, the discrepacy
>between the manpage and the code is also due to the lack of updates on the
>"MPOL_MF_STRICT is ignored on huge page mappings." statement.
>
>On Wed, Jan 15, 2020 at 01:59:14PM -0800, Yang Shi wrote:
>>
>> On 1/15/20 1:45 PM, Mike Kravetz wrote:
>> > On 1/15/20 1:30 PM, Yang Shi wrote:
>> > > On 1/15/20 1:07 PM, Mike Kravetz wrote:
>> > > > What should we do?
>> > > > ==================
>> > > > 1) Nothing more than optimizations by Li Xinhai.  Behavior that could be
>> > > >      seen as conflicting with man page has existed since v3.12 and I am
>> > > >      not aware of any complaints.
>> > > > 2) In addition to optimizations by Li Xinhai, modify code to truly ignore
>> > > >      MPOL_MF_STRICT for huge page mappings.  This would be fairly easy to do
>> > > >      after a failure of migrate_pages().  We could simply traverse the list
>> > > >      of pages that were not migrated looking for any non-hugetlb page.
>> > > I don't think we can do this easily since migrate_pages() would put the migration failed hugetlb pages back to hugepage_activelist so there should not any hugetlb page reside on the pagelist regardless of failure if I read the code correctly.
>
>Although this behavior seems to me not prevent from finding non-hugetlb
>pages in migration list, this is a difference in migration behavior between
>normal pages and hugepages that might be better to be optimized.
>Maybe hugepages failed to migrate should remain in migration list after
>migrate_pages() returns and the should be put back via putback_movable_pages().
>
>> > >
>> > You are correct.  I made an assumption without actually looking at the code. :(
>> >
>> > > Other than that traversing page list to look for a certain type page doesn't sound scalable to me.
>> > >
>> > > > 3) Remove the statement "MPOL_MF_STRICT is ignored on huge page mappings."
>> > > >      and modify code accordingly.
>> > > >
>> > > > My suggestion would be for 1 or 2.  Thoughts?
>> > > By rethinking the history (thanks again for digging into it), it sounds #3 should be more reasonable. It sounds like the behavior was changed since hugetlb migration was added but the man page was not updated to reflect the change.
>> > >
>> > Let's hope Naoya comments.  My only concern with #3 is that we will be changing
>> > behavior.  I do not think many people (if any) depend on existing behavior.
>> > However, you can never be sure.
>>
>> Yes, this would change the bahavior, but I don't see why we have to treat
>> hugetlb specially nowadays with migration supported.
>
>(Option #1 is good for short term solution, but eventually) I agree with option #3.
>We have no reason to handle hugetlb differently about MPOL_MF_STRICT flag. 

Thanks. Same thoughts for option #3, but it seems better not change current
behavior. 
Add more about current behavior of code:
- In unmap&move phase, there is no different behavior of handling hugepage
and non-hugepage, that is when STRICT is set, report -EIO if any page
can't move, when STRICT is not set, don't report when failed to move;
- In isolation phase, STRICT is effective for non-hugepge, that means set
STRICT alone will cause -EIO if found misplaced pages, and set STRICT with
MOVE* will cause -EIO if failed to isolate pages; for hugepage, STRICT is
ignored, it don't detect misplaced pages nor report -EIO if isolation failed.

This patch don't change any part of current behavior, only avoids walking
page table, where currently do nothing if STRICT is set alone.

>
>Thanks,
>Naoya Horiguchi