linux-kernel - [PATCHv2 11/14] hugetlb: Remove VMEMMAP_SYNCHRONIZE

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Message-ID: <20251218150949.721480-12-kas@kernel.org>
Date: Thu, 18 Dec 2025 15:09:42 +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 11/14] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU

The VMEMMAP_SYNCHRONIZE_RCU flag triggered synchronize_rcu() calls to
prevent a race between HVO remapping and page_ref_add_unless(). The
race could occur when a speculative PFN walker tried to modify the
refcount on a struct page that was in the process of being remapped
to a fake head.

With fake heads eliminated, page_ref_add_unless() no longer needs RCU
protection.

Remove the flag and synchronize_rcu() calls.

Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
---
 mm/hugetlb_vmemmap.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 63d79ac80594..cc0fcf847810 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -48,8 +48,6 @@ struct vmemmap_remap_walk {
 #define VMEMMAP_SPLIT_NO_TLB_FLUSH	BIT(0)
 /* Skip the TLB flush when we remap the PTE */
 #define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(1)
-/* synchronize_rcu() to avoid writes from page_ref_add_unless() */
-#define VMEMMAP_SYNCHRONIZE_RCU		BIT(2)
 	unsigned long		flags;
 };
 
@@ -423,9 +421,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
 	if (!folio_test_hugetlb_vmemmap_optimized(folio))
 		return 0;
 
-	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
-		synchronize_rcu();
-
 	vmemmap_start	= (unsigned long)folio;
 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 
@@ -456,7 +451,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
  */
 int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
 {
-	return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
+	return __hugetlb_vmemmap_restore_folio(h, folio, 0);
 }
 
 /**
@@ -479,14 +474,11 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
 	struct folio *folio, *t_folio;
 	long restored = 0;
 	long ret = 0;
-	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
 
 	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
 		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
 			ret = __hugetlb_vmemmap_restore_folio(h, folio, flags);
-			/* only need to synchronize_rcu() once for each batch */
-			flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
-
 			if (ret)
 				break;
 			restored++;
@@ -576,8 +568,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 
 	static_branch_inc(&hugetlb_optimize_vmemmap_key);
 
-	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
-		synchronize_rcu();
 	/*
 	 * Very Subtle
 	 * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
@@ -636,7 +626,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
 {
 	LIST_HEAD(vmemmap_pages);
 
-	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
+	__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
 	free_vmemmap_page_list(&vmemmap_pages);
 }
 
@@ -664,7 +654,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
 	struct folio *folio;
 	int nr_to_optimize;
 	LIST_HEAD(vmemmap_pages);
-	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
+	unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
 
 	nr_to_optimize = 0;
 	list_for_each_entry(folio, folio_list, lru) {
@@ -717,8 +707,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
 		int ret;
 
 		ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
-		/* only need to synchronize_rcu() once for each batch */
-		flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
 
 		/*
 		 * Pages to be freed may have been accumulated.  If we
-- 
2.51.2