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
