From nobody Sat Feb 7 15:06:03 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 F203B26F29B for ; Mon, 2 Feb 2026 15:58:22 +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=1770047903; cv=none; b=VRH6byvVfPqffGYfMAUZ828olOc9zZbjPbj/eYgKIk9Z4l44JE+qokI7p4nPPYtzDSIyN+/BoxgqhBMCSVQ79QaleVsppheq4XFmG+Wgoja5KkncFzXOhziYGJ7+AtheZEk0y64/E0KvBvLPdwexX6jiLxTOGjftjJI5TbnFg8E= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770047903; c=relaxed/simple; bh=7Z6tJ7UV3KGjuuHR1mbB3/Q82XEIqePwo/xEe3Pwwms=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=L8a1wXvq/I6Gd7sh2IYxTAxMdOFReEAocJvbnLz4azmRG5pOy4ScUsHHT8Pax6n2EF7Qgfw7D1ZQmtNsP5vho5gyRD07vv3PVSxMVlQc07AIYX54vVGAXpmvWLiwn6qR7TuKhJDb0o9S8YAIAn8Ma5zStytbC7/IbX5ODN/fAas= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fwRkykUT; 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="fwRkykUT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DEC80C116C6; Mon, 2 Feb 2026 15:58:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770047902; bh=7Z6tJ7UV3KGjuuHR1mbB3/Q82XEIqePwo/xEe3Pwwms=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fwRkykUTveFdtHDDtWZ17IxHYNcVVg6JFrPDloSIVKjKNBj7HwDrWI1NN5uTxlTn8 nuQsk+TgJyrxjif31zS6MXE+fCBKsIgal1xxQHPcmdbJ9kORUWBVGCmAcamTvOBTO4 Vwjt6tb03/fCsZ3sCLECiVfqYajNTJQEiQA+fBgj/eZ6udTRoMSSfF+vIuFnPIC1vW XTvjdQlf9kGIR6diRgzM+OQnMMA/S9ruyuyKYqImXNdzQBzXdnI+7WTIdsaK9qjgB9 ZvDBhhr9DllPmu07lbPaLHXZHU/9eDIN7T6D3I/jiHGFUtw/OIrvsWtbkuokjoa+UV zqD4a1vuJLJgw== Received: from phl-compute-01.internal (phl-compute-01.internal [10.202.2.41]) by mailfauth.phl.internal (Postfix) with ESMTP id 10639F4006A; Mon, 2 Feb 2026 10:58:21 -0500 (EST) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-01.internal (MEProxy); Mon, 02 Feb 2026 10:58:21 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgddujeektdeiucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujf gurhephffvvefufffkofgjfhgggfestdekredtredttdenucfhrhhomhepmfhirhihlhcu ufhhuhhtshgvmhgruhcuoehkrghssehkvghrnhgvlhdrohhrgheqnecuggftrfgrthhtvg hrnhephfdufeejhefhkedtuedvfeevjeffvdfhvedtudfgudffjeefieekleehvdetvdev necuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmrghilhhfrhhomhepkhhirh hilhhlodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdduieduudeivdeiheeh qddvkeeggeegjedvkedqkhgrsheppehkvghrnhgvlhdrohhrghesshhhuhhtvghmohhvrd hnrghmvgdpnhgspghrtghpthhtohepvdekpdhmohguvgepshhmthhpohhuthdprhgtphht thhopegrkhhpmheslhhinhhugidqfhhouhhnuggrthhiohhnrdhorhhgpdhrtghpthhtoh epmhhutghhuhhnrdhsohhngheslhhinhhugidruggvvhdprhgtphhtthhopegurghvihgu sehrvgguhhgrthdrtghomhdprhgtphhtthhopeifihhllhihsehinhhfrhgruggvrggurd horhhgpdhrtghpthhtohepuhhsrghmrggrrhhifheigedvsehgmhgrihhlrdgtohhmpdhr tghpthhtohepfhhvughlsehgohhoghhlvgdrtghomhdprhgtphhtthhopehoshgrlhhvrg guohhrsehsuhhsvgdruggvpdhrtghpthhtoheprhhpphhtsehkvghrnhgvlhdrohhrghdp rhgtphhtthhopehvsggrsghkrgesshhushgvrdgtii X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 2 Feb 2026 10:58:19 -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 12/17] mm: Drop fake head checks Date: Mon, 2 Feb 2026 15:56:28 +0000 Message-ID: <20260202155634.650837-13-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> 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" With fake head pages eliminated in the previous commit, remove the supporting infrastructure: - page_fixed_fake_head(): no longer needed to detect fake heads; - page_is_fake_head(): no longer needed; - page_count_writable(): no longer needed for RCU protection; - RCU read_lock in page_ref_add_unless(): no longer needed; This substantially simplifies compound_head() and page_ref_add_unless(), removing both branches and RCU overhead from these hot paths. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Acked-by: David Hildenbrand (Arm) --- include/linux/page-flags.h | 93 ++------------------------------------ include/linux/page_ref.h | 8 +--- 2 files changed, 4 insertions(+), 97 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 8f2c7fbc739b..5a8f6fab2255 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -221,102 +221,15 @@ static __always_inline bool compound_info_has_mask(v= oid) return is_power_of_2(sizeof(struct page)); } =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 compound_info_has_mask() is true */ - if (!compound_info_has_mask()) - 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); - - /* See set_compound_head() */ - 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 compound_info_has_mask() is false, the rest of compound_info is @@ -397,7 +310,7 @@ 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) @@ -924,7 +837,7 @@ 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