Re: [PATCH -next 1/7] mm_types: add _last_cpupid into folio

[Huang, Ying]

Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> writes:

> On 2023/10/10 20:33, Matthew Wilcox wrote:
>> On Tue, Oct 10, 2023 at 02:45:38PM +0800, Kefeng Wang wrote:
>>> At present, only arc/sparc/m68k define WANT_PAGE_VIRTUAL, both of
>>> them don't support numa balancing, and the page struct is aligned
>>> to _struct_page_alignment, it is safe to move _last_cpupid before
>>> 'virtual' in page, meanwhile, add it into folio, which make us to
>>> use folio->_last_cpupid directly.
>> What do you mean by "safe"?  I think you mean "Does not increase the
>> size of struct page", but if that is what you mean, why not just say so?
>> If there's something else you mean, please explain.
>
> Don't increase size of struct page and don't impact the real order of
> struct page as the above three archs without numa balancing support.
>
>> In any event, I'd like to see some reasoning that _last_cpupid is
>> actually
>> information which is logically maintained on a per-allocation basis,
>> not a per-page basis (I think this is true, but I honestly don't know)
>
> The _last_cpupid is updated in should_numa_migrate_memory() from numa
> fault(do_numa_page, and do_huge_pmd_numa_page), it is per-page(normal
> page and PMD-mapped page). Maybe I misunderstand your mean, please
> correct me.

Because PTE mapped THP will not be migrated according to comments and
folio_test_large() test in do_numa_page().  Only _last_cpuid of the head
page will be used (that is, on per-allocation basis).  Although in
change_pte_range() in mprotect.c, _last_cpuid of tail pages may be
changed, they are not used actually.  All in all, _last_cpuid is on
per-allocation basis for now.

In the future, it's hard to say.  PTE-mapped THPs or large folios give
us an opportunity to check whether the different parts of a folio are
accessed by multiple sockets, so that we should split the folio.  But
this is just some possibility in the future.

--
Best Regards,
Huang, Ying