From nobody Sun Feb 8 12:37:10 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7FCA83431E9 for ; Thu, 18 Dec 2025 15:10:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766070615; cv=none; b=XAmKi1K9nOgLSX239xmLXJAoC4Aq5k1Wxsgcb6Tj7af7UXQQQ418XMHQCfaLQSbvGzsAak6aAbFO9X8dH0wgvqx3M/wFjs580zxYY+vKzb1IOwPAdQLtW4WfAQYBh4JmmxnauLNm/8cyZNMiOpHWqJA8OMzDwwwAqf344+VxNXo= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766070615; c=relaxed/simple; bh=B1eibsAcWSn5y/4mdCGhgsGHb7qIzuBZJfdR09u3QuM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=DY9/eXKxfPatQLdpcVvmPGRnRrtd6dymjk1Rmr2P2++BJ82BCCcdrB6UEWlx9ZjtSEUrMZmo+JCZmCN9QuMWzkGFp/Dwdq34zl3MrdCx4U1IlBsblTtsUk07jRha0KVggWfDanxnVlYJk2nSscHkm21tzJ3vDVhkVoo3gJFU7e0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LJacVhkQ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LJacVhkQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 70D3CC116B1; Thu, 18 Dec 2025 15:10:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766070615; bh=B1eibsAcWSn5y/4mdCGhgsGHb7qIzuBZJfdR09u3QuM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=LJacVhkQPLYFRT+bJ2vM8A7HwCIPArlwYIv8IJtOmXYgoFjQKETVuVklGANvu1pWJ rjkurIpKERJo3LRtBfokpqtUdnUKRuqigGEJYwt1aGJ6CI20p9bX63RoMqbjm84vDj pOz8cTNCA+X3oAoQ0az0VNHtP+J7c7zaPyXyDejvuKwDozFH12ePc2dVYcRLE19CGj f2PQ4k9V93mrNb7gW+zSQSU6u5Ouvm/E6/htNWqfWwZ2ZTp4rWmJ27ERTswS5pshRI bWjBcMS+F429gPSPQaUlBFeQzb7472LRdcuEnkTN2XDAACRcwRe2D5tLsJg+qG9lji x8J3f7GY2uiyg== Received: from phl-compute-04.internal (phl-compute-04.internal [10.202.2.44]) by mailfauth.phl.internal (Postfix) with ESMTP id C101CF40078; Thu, 18 Dec 2025 10:10:13 -0500 (EST) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-04.internal (MEProxy); Thu, 18 Dec 2025 10:10:13 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgdegheejhecutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpuffrtefokffrpgfnqfghnecuuegr ihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjug hrpefhvfevufffkffojghfggfgsedtkeertdertddtnecuhfhrohhmpefmihhrhihlucfu hhhuthhsvghmrghuuceokhgrsheskhgvrhhnvghlrdhorhhgqeenucggtffrrghtthgvrh hnpefhudefjeehhfektdeuvdefveejffdvhfevtddugfduffejfeeikeelhedvtedvveen ucevlhhushhtvghrufhiiigvpedunecurfgrrhgrmhepmhgrihhlfhhrohhmpehkihhrih hllhdomhgvshhmthhprghuthhhphgvrhhsohhnrghlihhthidqudeiudduiedvieehhedq vdekgeeggeejvdekqdhkrghspeepkhgvrhhnvghlrdhorhhgsehshhhuthgvmhhovhdrnh grmhgvpdhnsggprhgtphhtthhopedvtddpmhhouggvpehsmhhtphhouhhtpdhrtghpthht oheprghkphhmsehlihhnuhigqdhfohhunhgurghtihhonhdrohhrghdprhgtphhtthhope hmuhgthhhunhdrshhonhhgsehlihhnuhigrdguvghvpdhrtghpthhtohepuggrvhhiuges khgvrhhnvghlrdhorhhgpdhrtghpthhtohepfihilhhlhiesihhnfhhrrgguvggrugdroh hrghdprhgtphhtthhopehushgrmhgrrghrihhfieegvdesghhmrghilhdrtghomhdprhgt phhtthhopehfvhgulhesghhoohhglhgvrdgtohhmpdhrtghpthhtohepohhsrghlvhgrug horhesshhushgvrdguvgdprhgtphhtthhopehrphhptheskhgvrhhnvghlrdhorhhgpdhr tghpthhtohepvhgsrggskhgrsehsuhhsvgdrtgii X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Thu, 18 Dec 2025 10:10:13 -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: [PATCHv2 11/14] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Thu, 18 Dec 2025 15:09:42 +0000 Message-ID: <20251218150949.721480-12-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20251218150949.721480-1-kas@kernel.org> References: <20251218150949.721480-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 63d79ac80594..cc0fcf847810 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; }; =20 @@ -423,9 +421,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, if (!folio_test_hugetlb_vmemmap_optimized(folio)) return 0; =20 - if (flags & VMEMMAP_SYNCHRONIZE_RCU) - synchronize_rcu(); - vmemmap_start =3D (unsigned long)folio; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); =20 @@ -456,7 +451,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, */ int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *fo= lio) { - return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU); + return __hugetlb_vmemmap_restore_folio(h, folio, 0); } =20 /** @@ -479,14 +474,11 @@ long hugetlb_vmemmap_restore_folios(const struct hsta= te *h, struct folio *folio, *t_folio; long restored =3D 0; long ret =3D 0; - unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_= RCU; + unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH; =20 list_for_each_entry_safe(folio, t_folio, folio_list, lru) { if (folio_test_hugetlb_vmemmap_optimized(folio)) { ret =3D __hugetlb_vmemmap_restore_folio(h, folio, flags); - /* only need to synchronize_rcu() once for each batch */ - flags &=3D ~VMEMMAP_SYNCHRONIZE_RCU; - if (ret) break; restored++; @@ -576,8 +568,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, =20 static_branch_inc(&hugetlb_optimize_vmemmap_key); =20 - if (flags & VMEMMAP_SYNCHRONIZE_RCU) - synchronize_rcu(); /* * Very Subtle * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed @@ -636,7 +626,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate= *h, struct folio *folio) { LIST_HEAD(vmemmap_pages); =20 - __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHR= ONIZE_RCU); + __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0); free_vmemmap_page_list(&vmemmap_pages); } =20 @@ -664,7 +654,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hs= tate *h, struct folio *folio; int nr_to_optimize; LIST_HEAD(vmemmap_pages); - unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_= RCU; + unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH; =20 nr_to_optimize =3D 0; list_for_each_entry(folio, folio_list, lru) { @@ -717,8 +707,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hs= tate *h, int ret; =20 ret =3D __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags= ); - /* only need to synchronize_rcu() once for each batch */ - flags &=3D ~VMEMMAP_SYNCHRONIZE_RCU; =20 /* * Pages to be freed may have been accumulated. If we --=20 2.51.2