Re: [PATCH 2/2] mm: support multi-size THP numa balancing

[David Hildenbrand]

On 27.03.24 09:21, Huang, Ying wrote:
> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>
> > On 2024/3/27 10:04, Huang, Ying wrote:
> > > Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
> > >
> > > > Now the anonymous page allocation already supports multi-size THP (mTHP),
> > > > but the numa balancing still prohibits mTHP migration even though it is an
> > > > exclusive mapping, which is unreasonable.
> > > >
> > > > Allow scanning mTHP:
> > > > Commit 859d4adc3415 ("mm: numa: do not trap faults on shared data section
> > > > pages") skips shared CoW pages' NUMA page migration to avoid shared data
> > > > segment migration. In addition, commit 80d47f5de5e3 ("mm: don't try to
> > > > NUMA-migrate COW pages that have other uses") change to use page_count()
> > > > to avoid GUP pages migration, that will also skip the mTHP numa scaning.
> > > > Theoretically, we can use folio_maybe_dma_pinned() to detect the GUP
> > > > issue, although there is still a GUP race, the issue seems to have been
> > > > resolved by commit 80d47f5de5e3. Meanwhile, use the folio_likely_mapped_shared()
> > > > to skip shared CoW pages though this is not a precise sharers count. To
> > > > check if the folio is shared, ideally we want to make sure every page is
> > > > mapped to the same process, but doing that seems expensive and using
> > > > the estimated mapcount seems can work when running autonuma benchmark.
> > > Because now we can deal with shared mTHP, it appears even possible
> > > to
> > > remove folio_likely_mapped_shared() check?
> >
> > IMO, the issue solved by commit 859d4adc3415 is about shared CoW
> > mapping, and I prefer to measure it in another patch:)
>
> I mean we can deal with shared mTHP (by multiple threads or multiple
> processes) with this patch.  Right?

It's independent of the folio order. We don't want to mess with shared COW pages, see

commit 859d4adc3415a64ccb8b0c50dc4e3a888dcb5805
Author: Henry Willard <henry.willard@xxxxxxxxxx>
Date:   Wed Jan 31 16:21:07 2018 -0800

    mm: numa: do not trap faults on shared data section pages.
    
    Workloads consisting of a large number of processes running the same
    program with a very large shared data segment may experience performance
    problems when numa balancing attempts to migrate the shared cow pages.
    This manifests itself with many processes or tasks in
    TASK_UNINTERRUPTIBLE state waiting for the shared pages to be migrated.
..

that introduced this handling.

--
Cheers,

David / dhildenb