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

[Matthew Wilcox]

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.

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)

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.