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

[Kefeng Wang]

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.

> And looking at all this, I think it makes sense to move _last_cpupid
> before the kmsan garbage, then add both 'virtual' and '_last_cpupid'
> to folio.

sure, I will add both of them into folio and don't re-order 'virtual' 
and '_last_cpupid'.
>