linux-kernel - [PATCHv2 02/14] mm/sparse: Check memmap alignment

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Message-ID: <20251218150949.721480-3-kas@kernel.org>
Date: Thu, 18 Dec 2025 15:09:33 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	David Hildenbrand <david@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Usama Arif <usamaarif642@...il.com>,
	Frank van der Linden <fvdl@...gle.com>
Cc: Oscar Salvador <osalvador@...e.de>,
	Mike Rapoport <rppt@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Zi Yan <ziy@...dia.com>,
	Baoquan He <bhe@...hat.com>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Jonathan Corbet <corbet@....net>,
	kernel-team@...a.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Kiryl Shutsemau <kas@...nel.org>
Subject: [PATCHv2 02/14] mm/sparse: Check memmap alignment

The upcoming changes in compound_head() require memmap to be naturally
aligned to the maximum folio size.

Add a warning if it is not.

A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the
kernel is still likely to be functional if this strict check fails.

Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
---
 include/linux/mmzone.h | 1 +
 mm/sparse.c            | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 6cfede39570a..9f44dc760cdc 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -91,6 +91,7 @@
 #endif
 
 #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
+#define MAX_FOLIO_SIZE		(PAGE_SIZE << MAX_FOLIO_ORDER)
 
 enum migratetype {
 	MIGRATE_UNMOVABLE,
diff --git a/mm/sparse.c b/mm/sparse.c
index 17c50a6415c2..c5810ff7c6f7 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -600,6 +600,9 @@ void __init sparse_init(void)
 	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
 	memblocks_present();
 
+	WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0),
+			    MAX_FOLIO_SIZE / sizeof(struct page)));
+
 	pnum_begin = first_present_section_nr();
 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
 
-- 
2.51.2