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Fri, 27 Feb 2026 14:43:31 -0500 (EST) From: "Kiryl Shutsemau (Meta)" 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 , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, Kiryl Shutsemau Subject: [PATCHv7 14/18] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Fri, 27 Feb 2026 19:42:52 +0000 Message-ID: <20260227194302.274384-15-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260227194302.274384-1-kas@kernel.org> References: <20260227194302.274384-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 11B7A120013 X-Stat-Signature: ntj3upnhnmut1azuajrym5fmdgo6xsde X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1772221414-634225 X-HE-Meta: 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 Qmt9J3bq nuHVnP2vIHRLMqU5ExFegJRHeeBQS1NNcp9hkaQLycP4965boOu/3LkDZ5LSatgKZvp5Cx25zqFLUiad9lt4jD5xKfrDA5HkJe9hofMGVinZEDRZsFYqNeRd+WshP8lMhKgatK5ZDXlc4UgrsDC7umlRcuDMIRfEWQfwoCB7ylAryq/F+loNDmuqPafQthNaSolSB2dN7ioECVUGMeyp7FWxeTPJujFOnfjUbC1LDkzdAH3qFMfntX7CA0wbAPfUCtBSOPU4OSkOQ+BRoYSTZizuYBN6m8IZD5eXAzOg97/58lqgTqW2p8pffHQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Kiryl Shutsemau 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 Reviewed-by: David Hildenbrand (Arm) --- 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 92330f172eb7..fd1d5d5d12b4 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -47,8 +47,6 @@ 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; }; @@ -409,9 +407,6 @@ 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->page; vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h); @@ -444,7 +439,7 @@ 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); } /** @@ -467,14 +462,11 @@ 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++; @@ -554,8 +546,6 @@ 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 @@ -613,7 +603,7 @@ 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); } @@ -641,7 +631,7 @@ 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) { @@ -694,8 +684,6 @@ 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