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Thu, 15 Jan 2026 09:46:27 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv3 12/15] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Thu, 15 Jan 2026 14:45:58 +0000 Message-ID: <20260115144604.822702-13-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260115144604.822702-1-kas@kernel.org> References: <20260115144604.822702-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 389yx951rqoc73aspdiu4qy3fca55wxx X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 1230F14000F X-HE-Tag: 1768488389-567591 X-HE-Meta: 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 iv2hEXwj dSKVNClgHOCljk2o+sqowwFV9NBvyadAkvcGSguGCioyaS/bPRlj4E1o/WE+7dN8yBAZidQpMSxNtb5S6Nu+JJsY052m1d97Xxs3wRhPWCvBfeMAulDcZnj3JmRuzKx7WEGQwsmTTYhRwV9JwhkgbtzVRDkQtynkLLBty5nMx5AiWJYJ6AVexEn1bqRVCgeBUloKlmXAtWBRvwaJVMXJVnDQ2oQJNWn6BJTEnF/B8di6TVgHiuf8KvoDQj+4l7bfixIwswXtwo4RJwae/515mtbCXoNIU1CF23HOUz1ROg4FvSFa0fN+kL2Ohwg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Reviewed-by: Muchun Song --- 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 cbdca4684db1..5a802e292bac 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