Re: [PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

["David Hildenbrand (Red Hat)" <davidhildenbrandkernel@gmail.com>]

On 10.11.25 07:37, Anshuman Khandual wrote:
> struct ptdesc (including all relevant helpers) support multi order compound
> pages. But page_ptdesc() coverts given page into its own ptdesc rather than
> deriving from its compound head as would have been expected otherwise. Just
> change the macro to fetch the struct ptdesc from the compound head instead,
> so that the same struct ptdesc is reached from all tail pages.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v6.18-rc5
> 
> Found via code inspection. Apparently struct ptdesc could represent a page
> table page which is multi order looking into helpers as ptdesc_nr_pages(),
> __pagetable_ctor/dtor() and pagetable_free() etc. Am I missing something ?
> 
>   include/linux/mm_types.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 90e5790c318f..f7107bc55d1e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -604,9 +604,9 @@ static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
>   	const struct ptdesc *:		(const struct folio *)(pt),	\
>   	struct ptdesc *:		(struct folio *)(pt)))
>   
> -#define page_ptdesc(p)			(_Generic((p),			\
> -	const struct page *:		(const struct ptdesc *)(p),	\
> -	struct page *:			(struct ptdesc *)(p)))
> +#define page_ptdesc(p)			(_Generic((p),					\
> +	const struct page *:		(const struct ptdesc *)_compound_head(p),	\
> +	struct page *:			(struct ptdesc *)_compound_head(p)))

Well, this adds overhead :)

The real question is when we would be converting from a tail page to a 
ptdesc.

Take a look at pmd_ptdesc()->pmd_pgtable_page() where we avoid looking 
up a tail page in the first place.