linux-kernel - [PATCH v1 09/36] mm/mm_init: make memmap_init_compound() look more like prep_compound

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Message-ID: <20250827220141.262669-10-david@redhat.com>
Date: Thu, 28 Aug 2025 00:01:13 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: David Hildenbrand <david@...hat.com>,
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
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Subject: [PATCH v1 09/36] mm/mm_init: make memmap_init_compound() look more like prep_compound_page()

Grepping for "prep_compound_page" leaves on clueless how devdax gets its
compound pages initialized.

Let's add a comment that might help finding this open-coded
prep_compound_page() initialization more easily.

Further, let's be less smart about the ordering of initialization and just
perform the prep_compound_head() call after all tail pages were
initialized: just like prep_compound_page() does.

No need for a comment to describe the initialization order: again,
just like prep_compound_page().

Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/mm_init.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 5c21b3af216b2..df614556741a4 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1091,6 +1091,12 @@ static void __ref memmap_init_compound(struct page *head,
 	unsigned long pfn, end_pfn = head_pfn + nr_pages;
 	unsigned int order = pgmap->vmemmap_shift;
 
+	/*
+	 * We have to initialize the pages, including setting up page links.
+	 * prep_compound_page() does not take care of that, so instead we
+	 * open-code prep_compound_page() so we can take care of initializing
+	 * the pages in the same go.
+	 */
 	__SetPageHead(head);
 	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
 		struct page *page = pfn_to_page(pfn);
@@ -1098,15 +1104,8 @@ static void __ref memmap_init_compound(struct page *head,
 		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
 		prep_compound_tail(head, pfn - head_pfn);
 		set_page_count(page, 0);
-
-		/*
-		 * The first tail page stores important compound page info.
-		 * Call prep_compound_head() after the first tail page has
-		 * been initialized, to not have the data overwritten.
-		 */
-		if (pfn == head_pfn + 1)
-			prep_compound_head(head, order);
 	}
+	prep_compound_head(head, order);
 }
 
 void __ref memmap_init_zone_device(struct zone *zone,
-- 
2.50.1