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Wed, 21 Jan 2026 11:23:18 -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: [PATCHv4 11/14] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Wed, 21 Jan 2026 16:22:48 +0000 Message-ID: <20260121162253.2216580-12-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 23A8E18000B X-Stat-Signature: 9t5ztpi9c3ffop6e8ihrr7f8mce1q4zy X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1769012600-499096 X-HE-Meta: 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 yvAtGu0j kMR/wbUvAOooFWY7ppRSExCF5Kj1xACdB2D93BZaRXkMNNMGO5sGPkzG1MLvg5rifgd43XLh6zqmwRGvmyUdG/J4+LYz8B5t2TGNi5yr8btTEYh0mVeN+I2GFTVdF+ilAUFcFRW+FUT8pne0osZhHerNVXLXDR84e+Xor+RXLRvd6LUG0bAnJrOlB9mD9//CJmMUXx5Z/Edb41VBkcneQkg56NL/JwhN1ZALq+gvLBl1ci8UgWbBfo3mr4V7KeCdsa/YrHC023c+DQNh/EOYi0/Hyd3SVX3kEcIvqXDRJESsZDX0F5TwHjPQp0Q== 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 51bb6c73db92..f2a3340c8e22 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; }; @@ -410,9 +408,6 @@ 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); vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE; @@ -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++; @@ -564,8 +556,6 @@ 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 @@ -624,7 +614,7 @@ 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); } @@ -652,7 +642,7 @@ 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) { @@ -705,8 +695,6 @@ 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