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

[David Hildenbrand <david@redhat.com>]

On 23.10.24 13:38, Kirill A. Shutemov wrote:
> 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@redhat.com>
> 
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 
> 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.

I recall that Willy did, and it's mostly getting rid of a single 
instruction in loads of places.

$ git grep folio_nr_pages | wc -l
254


[my first intuition was also to just remove it, but this way seems easy 
to just maintain it for now]

-- 
Cheers,

David / dhildenb