From nobody Sat Feb 7 14:08:43 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 7CE19345CDA for ; Fri, 5 Dec 2025 19:44:07 +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=1764963847; cv=none; b=Qnq959S4Z7hBftz2doXoZcbDkGIRKVSN4Z0tQmTaisgGMe6pJcCWHI4nqUl1na1tnjRKyAkNvJSmCStwRjpzOT1nK+CPv2oQF7u1ieri2SgSmqnwSOXPdKH4dcdQnlHh6AJgPSMLE/9uXw26TQcTqqDPobBINke4LROnxQ4TCTg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764963847; c=relaxed/simple; bh=QSFmiqQuAu4Xe8qwQuUx5JcV9uyzyjf22IqZdZAyLHc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=eHDtUd7q2+QcjhvaqAt4hmvx5zIv51cb4y+P/7+R3ZtcrbMWnokpG1QvUoxjgd6qHsRv0VaGNL3QJxSpvS8eFIskok5TLFFYwUF/y/ZmgwL/24pJUNhx/TgyWf6FqM0QciFVUX9nN0mMkX7QHbEwLIeuUgtx7nZ/RUOwDZQ5K58= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YYBZl+RF; 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="YYBZl+RF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3D725C4AF0B; Fri, 5 Dec 2025 19:44:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764963846; bh=QSFmiqQuAu4Xe8qwQuUx5JcV9uyzyjf22IqZdZAyLHc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=YYBZl+RFV5ksiVH6NISnSSMBo/mhcOKzJuEWY5Pp+wC7asR3BN6qPaFjNKjZfrimC K8OxiULVd3Yn2ye9Kfr2j5ecodkEZkAENl+M9xuunlVPEAxyIJlCD9H9iFNSh2rske JeX8z5NuAJejFQkx8ckt4LKSzQevIAOEXOv5GfQagVMATh0/ZKqPVbAzmnvgRJglHx YWcygjV0yCovSob3ppuLwR6tFh6Kf3KGYH9BdCogYM+QYfBL+A5TyHkuZmBqQTLGEk rydEW+5V/WMIaLl6G/N8ojQTR3WaK/6Q/y+9MBHtO39pi+znAircJ+QRACAUp52FZ+ VTMBULgDSVbqg== Received: from phl-compute-12.internal (phl-compute-12.internal [10.202.2.52]) by mailfauth.phl.internal (Postfix) with ESMTP id 882CBF40070; Fri, 5 Dec 2025 14:44:05 -0500 (EST) Received: from phl-mailfrontend-01 ([10.202.2.162]) by phl-compute-12.internal (MEProxy); Fri, 05 Dec 2025 14:44:05 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgdelvdehucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceurghi lhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujfgurh ephffvvefufffkofgjfhgggfestdekredtredttdenucfhrhhomhepmfhirhihlhcuufhh uhhtshgvmhgruhcuoehkrghssehkvghrnhgvlhdrohhrgheqnecuggftrfgrthhtvghrnh ephfdufeejhefhkedtuedvfeevjeffvdfhvedtudfgudffjeefieekleehvdetvdevnecu vehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmrghilhhfrhhomhepkhhirhhilh hlodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdduieduudeivdeiheehqddv keeggeegjedvkedqkhgrsheppehkvghrnhgvlhdrohhrghesshhhuhhtvghmohhvrdhnrg hmvgdpnhgspghrtghpthhtohepudelpdhmohguvgepshhmthhpohhuthdprhgtphhtthho pegrkhhpmheslhhinhhugidqfhhouhhnuggrthhiohhnrdhorhhgpdhrtghpthhtohepmh hutghhuhhnrdhsohhngheslhhinhhugidruggvvhdprhgtphhtthhopegurghvihgusehk vghrnhgvlhdrohhrghdprhgtphhtthhopehoshgrlhhvrgguohhrsehsuhhsvgdruggvpd hrtghpthhtoheprhhpphhtsehkvghrnhgvlhdrohhrghdprhgtphhtthhopehvsggrsghk rgesshhushgvrdgtiidprhgtphhtthhopehlohhrvghniihordhsthhorghkvghssehorh grtghlvgdrtghomhdprhgtphhtthhopeifihhllhihsehinhhfrhgruggvrggurdhorhhg pdhrtghpthhtohepiihihiesnhhvihguihgrrdgtohhm X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Fri, 5 Dec 2025 14:44:05 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song Cc: David Hildenbrand , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Matthew Wilcox , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Usama Arif , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCH 07/11] mm: Drop fake head checks and fix a race condition Date: Fri, 5 Dec 2025 19:43:43 +0000 Message-ID: <20251205194351.1646318-8-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20251205194351.1646318-1-kas@kernel.org> References: <20251205194351.1646318-1-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" Fake heads are no longer in use, so checks for them should be removed. It simplifies compound_head() and page_ref_add_unless() substantially. Signed-off-by: Kiryl Shutsemau --- include/linux/page-flags.h | 95 ++------------------------------------ include/linux/page_ref.h | 8 +--- 2 files changed, 4 insertions(+), 99 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index eef02fbbb40f..8acb141a127b 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -198,104 +198,15 @@ enum pageflags { =20 #ifndef __GENERATING_BOUNDS_H =20 -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); =20 -/* - * Return the real head page struct iff the @page is a fake head page, oth= erwise - * return the @page itself. See Documentation/mm/vmemmap_dedup.rst. - */ -static __always_inline const struct page *page_fixed_fake_head(const struc= t page *page) -{ - if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) - return page; - - /* - * Fake heads only exists if size of struct page is power-of-2. - * See hugetlb_vmemmap_optimizable_size(). - */ - if (!is_power_of_2(sizeof(struct page))) - return page; - - /* - * Only addresses aligned with PAGE_SIZE of struct page may be fake head - * struct page. The alignment check aims to avoid access the fields ( - * e.g. compound_info) of the @page[1]. It can avoid touch a (possibly) - * cold cacheline in some cases. - */ - if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) && - test_bit(PG_head, &page->flags.f)) { - /* - * We can safely access the field of the @page[1] with PG_head - * because the @page is a compound page composed with at least - * two contiguous pages. - */ - unsigned long info =3D READ_ONCE(page[1].compound_info); - - if (likely(info & 1)) { - unsigned long p =3D (unsigned long)page; - - return (const struct page *)(p & info); - } - } - return page; -} - -static __always_inline bool page_count_writable(const struct page *page, i= nt u) -{ - if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) - return true; - - /* - * The refcount check is ordered before the fake-head check to prevent - * the following race: - * CPU 1 (HVO) CPU 2 (speculative PFN walker) - * - * page_ref_freeze() - * synchronize_rcu() - * rcu_read_lock() - * page_is_fake_head() is false - * vmemmap_remap_pte() - * XXX: struct page[] becomes r/o - * - * page_ref_unfreeze() - * page_ref_count() is not zero - * - * atomic_add_unless(&page->_refcount) - * XXX: try to modify r/o struct page[] - * - * The refcount check also prevents modification attempts to other (r/o) - * tail pages that are not fake heads. - */ - if (atomic_read_acquire(&page->_refcount) =3D=3D u) - return false; - - return page_fixed_fake_head(page) =3D=3D page; -} -#else -static inline const struct page *page_fixed_fake_head(const struct page *p= age) -{ - return page; -} - -static inline bool page_count_writable(const struct page *page, int u) -{ - return true; -} -#endif - -static __always_inline int page_is_fake_head(const struct page *page) -{ - return page_fixed_fake_head(page) !=3D page; -} - static __always_inline unsigned long _compound_head(const struct page *pag= e) { unsigned long info =3D READ_ONCE(page->compound_info); =20 /* Bit 0 encodes PageTail() */ if (!(info & 1)) - return (unsigned long)page_fixed_fake_head(page); + return (unsigned long)page; =20 /* * If the size of struct page is not power-of-2, the rest if @@ -377,7 +288,7 @@ static __always_inline void clear_compound_head(struct = page *page) =20 static __always_inline int PageTail(const struct page *page) { - return READ_ONCE(page->compound_info) & 1 || page_is_fake_head(page); + return READ_ONCE(page->compound_info) & 1; } =20 static __always_inline int PageCompound(const struct page *page) @@ -904,7 +815,7 @@ static __always_inline bool folio_test_head(const struc= t folio *folio) static __always_inline int PageHead(const struct page *page) { PF_POISONED_CHECK(page); - return test_bit(PG_head, &page->flags.f) && !page_is_fake_head(page); + return test_bit(PG_head, &page->flags.f); } =20 __SETPAGEFLAG(Head, head, PF_ANY) diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h index 544150d1d5fd..490d0ad6e56d 100644 --- a/include/linux/page_ref.h +++ b/include/linux/page_ref.h @@ -230,13 +230,7 @@ static inline int folio_ref_dec_return(struct folio *f= olio) =20 static inline bool page_ref_add_unless(struct page *page, int nr, int u) { - bool ret =3D false; - - rcu_read_lock(); - /* avoid writing to the vmemmap area being remapped */ - if (page_count_writable(page, u)) - ret =3D atomic_add_unless(&page->_refcount, nr, u); - rcu_read_unlock(); + bool ret =3D atomic_add_unless(&page->_refcount, nr, u); =20 if (page_ref_tracepoint_active(page_ref_mod_unless)) __page_ref_mod_unless(page, nr, ret); --=20 2.51.2