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Wed, 28 Jan 2026 08:56:54 -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 , 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: [PATCHv5 13/17] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Wed, 28 Jan 2026 13:54:54 +0000 Message-ID: <20260128135500.22121-14-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260128135500.22121-1-kas@kernel.org> References: <20260128135500.22121-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Stat-Signature: cqeued1dn1f3p71zreua4wu9qftpgpko X-Rspam-User: X-Rspamd-Queue-Id: 5F5A3100003 X-HE-Tag: 1769608618-210128 X-HE-Meta: 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 GUqkwf7I XLTPSLuA6i8dn0dnqKf48AbJTy8GgrOSSZVsQW7ZerWji4+SqskLUADccx5RqWVeltfw73hs+vHun6EhsJTvCzTZUwRCpD0TlkJ4HMUf5bzbcvw2KTmnp11+SJYdZJVKR8t6LhYBfkBqn+Buh282SutW+6pR43f0C5ZFDfdKlEE1J4UROKMH1mO8/9ROu/j+RcSu8SvvESg04Bv+dyPoU2faSBAT+MNCrWIBZuLWMnF9uQRlHkMDI9zqFlOzQs54ePRfna54id0Q5UbNx7BdGijGMsk0WV+Y1+fOX0w2K170OdGakpOWHLAf11Q== 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 f5f42b92dd7d..fd74a164c494 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++; @@ -556,8 +548,6 @@ 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 @@ -615,7 +605,7 @@ 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); } @@ -643,7 +633,7 @@ 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) { @@ -696,8 +686,6 @@ 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