linux-kernel - [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <20250821200701.1329277-11-david@redhat.com>
Date: Thu, 21 Aug 2025 22:06:36 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: David Hildenbrand <david@...hat.com>,
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Subject: [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()

All pages were already initialized and set to PageReserved() with a
refcount of 1 by MM init code.

In fact, by using __init_single_page(), we will be setting the refcount to
1 just to freeze it again immediately afterwards.

So drop the __init_single_page() and use __ClearPageReserved() instead.
Adjust the comments to highlight that we are dealing with an open-coded
prep_compound_page() variant.

Further, as we can now safely iterate over all pages in a folio, let's
avoid the page-pfn dance and just iterate the pages directly.

Note that the current code was likely problematic, but we never ran into
it: prep_compound_tail() would have been called with an offset that might
exceed a memory section, and prep_compound_tail() would have simply
added that offset to the page pointer -- which would not have done the
right thing on sparsemem without vmemmap.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/hugetlb.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d12a9d5146af4..ae82a845b14ad 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3235,17 +3235,14 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 					unsigned long start_page_number,
 					unsigned long end_page_number)
 {
-	enum zone_type zone = zone_idx(folio_zone(folio));
-	int nid = folio_nid(folio);
-	unsigned long head_pfn = folio_pfn(folio);
-	unsigned long pfn, end_pfn = head_pfn + end_page_number;
+	struct page *head_page = folio_page(folio, 0);
+	struct page *page = folio_page(folio, start_page_number);
+	unsigned long i;
 	int ret;
 
-	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
-		struct page *page = pfn_to_page(pfn);
-
-		__init_single_page(page, pfn, zone, nid);
-		prep_compound_tail((struct page *)folio, pfn - head_pfn);
+	for (i = start_page_number; i < end_page_number; i++, page++) {
+		__ClearPageReserved(page);
+		prep_compound_tail(head_page, i);
 		ret = page_ref_freeze(page, 1);
 		VM_BUG_ON(!ret);
 	}
@@ -3257,12 +3254,14 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
 {
 	int ret;
 
-	/* Prepare folio head */
+	/*
+	 * This is an open-coded prep_compound_page() whereby we avoid
+	 * walking pages twice by preparing+freezing them in the same go.
+	 */
 	__folio_clear_reserved(folio);
 	__folio_set_head(folio);
 	ret = folio_ref_freeze(folio, 1);
 	VM_BUG_ON(!ret);
-	/* Initialize the necessary tail struct pages */
 	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
 	prep_compound_head((struct page *)folio, huge_page_order(h));
 }
-- 
2.50.1