linux-kernel - [RFC PATCH 4/5] mm: only use struct page in compound_nr() and compound

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Message-ID: <20260130034818.472804-5-ziy@nvidia.com>
Date: Thu, 29 Jan 2026 22:48:17 -0500
From: Zi Yan <ziy@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>,
	David Hildenbrand <david@...nel.org>,
	Matthew Wilcox <willy@...radead.org>
Cc: Alistair Popple <apopple@...dia.com>,
	Balbir Singh <balbirs@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Jens Axboe <axboe@...nel.dk>,
	Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Nico Pache <npache@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>,
	Barry Song <baohua@...nel.org>,
	Lance Yang <lance.yang@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>,
	Brendan Jackman <jackmanb@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	io-uring@...r.kernel.org
Subject: [RFC PATCH 4/5] mm: only use struct page in compound_nr() and compound_order()

A compound page is not a folio. Using struct folio in compound_nr() and
compound_order() is misleading. Use struct page and refer to the right
subpage of a compound page to set compound page order. compound_nr() is
calculated using compound_order() instead of reading folio->_nr_pages.

Signed-off-by: Zi Yan <ziy@...dia.com>
---
 include/linux/mm.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f8a8fd47399c..f1c54d9f4620 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1428,11 +1428,9 @@ static inline unsigned long folio_large_nr_pages(const struct folio *folio)
  */
 static inline unsigned int compound_order(const struct page *page)
 {
-	const struct folio *folio = (struct folio *)page;
-
-	if (!test_bit(PG_head, &folio->flags.f))
+	if (!test_bit(PG_head, &page->flags.f))
 		return 0;
-	return folio_large_order(folio);
+	return page[1].flags.f & 0xffUL;
 }
 
 /**
@@ -2514,11 +2512,9 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
  */
 static inline unsigned long compound_nr(const struct page *page)
 {
-	const struct folio *folio = (struct folio *)page;
-
-	if (!test_bit(PG_head, &folio->flags.f))
+	if (!test_bit(PG_head, &page->flags.f))
 		return 1;
-	return folio_large_nr_pages(folio);
+	return 1 << compound_order(page);
 }
 
 /**
-- 
2.51.0