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Mon, 2 Feb 2026 10:58:26 -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: [PATCHv6 13/17] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Mon, 2 Feb 2026 15:56:29 +0000 Message-ID: <20260202155634.650837-14-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-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 01485180013 X-Stat-Signature: yh3ubpo3tfntbaqjhsedrqut4hefoixk X-Rspam-User: X-HE-Tag: 1770047910-858595 X-HE-Meta: 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 7oNXgnQs o1w1faiTqjX3OnmewrJKONZsfaokHQkcuGy3/2YxSs2RgspEisFnZFUaUOCjaDAFTGHK39IH5ofc2AcTonBKR3SCNQ85QHCieAihHjscQgQjY5Vyf4op/DRgNk7/g00BmL+0Q1/KofTzDbyvuvjJs/fDODNPQ/AcazQcqExDw/PiCnI7LAGjlta5q3+Bmqvyjkk5lXTz0ZAKZMswuIyBMFNAPoKRwMG/dhdAeFGpPJzFxjmxZkw8M7pilnDAtLxv/nmA3083B4iKT25AaxcOeDIP+fFeemT63btsxA7akwJckazkRKxoRlP4xlQ== 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 688764c52c72..6088fc77865c 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