Re: [PATCH v1 04/17] mm: let _folio_nr_pages overlay memcg_data in first tail page

[Kirill A. Shutemov]

On Thu, Aug 29, 2024 at 06:56:07PM +0200, David Hildenbrand wrote:
> Let's free up some more of the "unconditionally available on 64BIT"
> space in order-1 folios by letting _folio_nr_pages overlay memcg_data in
> the first tail page (second folio page). Consequently, we have the
> optimization now whenever we have CONFIG_MEMCG, independent of 64BIT.
> 
> We have to make sure that page->memcg on tail pages does not return
> "surprises". page_memcg_check() already properly refuses PageTail().
> Let's do that earlier in print_page_owner_memcg() to avoid printing
> wrong "Slab cache page" information. No other code should touch that
> field on tail pages of compound pages.
> 
> Reset the "_nr_pages" to 0 when splitting folios, or when freeing them
> back to the buddy (to avoid false page->memcg_data "bad page" reports).
> 
> Note that in __split_huge_page(), folio_nr_pages() would stop working
> already as soon as we start messing with the subpages.
> 
> Most kernel configs should have at least CONFIG_MEMCG enabled, even if
> disabled at runtime. 64byte "struct memmap" is what we usually have
> on 64BIT.
> 
> While at it, rename "_folio_nr_pages" to "_nr_pages".
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

BTW, have anybody evaluated how much (if anything) do we gain we a
separate _nr_pages field in struct folio comparing to calculating it
based on the order in _flags_1? Mask+shift should be pretty cheap.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov