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

[Anshuman Khandual]

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@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
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)))
 
 #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
 static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
-- 
2.30.2