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Fri, 27 Feb 2026 14:30:54 -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 , "David Hildenbrand (Arm)" Subject: [PATCHv7 14/18] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Fri, 27 Feb 2026 19:30:15 +0000 Message-ID: <20260227193030.272078-14-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260202155634.650837-1-kas@kernel.org> References: <20260202155634.650837-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: ntj3upnhnmut1azuajrym5fmdgo6xsde X-Rspamd-Queue-Id: 29D4EA000D X-Rspamd-Server: rspam03 X-HE-Tag: 1772220656-404601 X-HE-Meta: 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 WS41fSZD F8tHkWwVb60fKdKyqKyhKh39+0pEUnjU3yUDNgV9kY5v3OqvKMawf5aXGk7psm2flg+mGh2TUMqRKXqb4z6UxvJFI+zPo6nVMJ9cynNyUurOjCY4+pzE03JdFlNeKMub2eeaN/Ry6J17ARGP0mt9n+TxCvBJzBXqiX0qEUSVonqqMAmOU3YgzYzEtMbS69BCJVQYq1MM2Ndb6Cv5thM1Fbpppa4KpkvZHcx0ciwh44ED2GPcK0GLVW6TTFqOxY0PKy2zzqxYfMwlfTweI3d+qluY80zZS3KA+ksas3jEpor3jjvoNBPM+aFJ13g== 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