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Thu, 15 Jan 2026 09:46:26 -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 , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv3 11/15] mm: Drop fake head checks Date: Thu, 15 Jan 2026 14:45:57 +0000 Message-ID: <20260115144604.822702-12-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260115144604.822702-1-kas@kernel.org> References: <20260115144604.822702-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: rsjkgfbtd64xfb4aw5mt9tqahsxqomxw X-Rspamd-Queue-Id: 2E380160009 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1768488388-247810 X-HE-Meta: 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 JGhSZaaF cpbKU11f2q+O2A6naull0f0N6foFGabZreZZhrc1vUeeO/7SowA+3zm4yS2VyRD+GwjuEDbH2DEiZPrPENdKNmMDGVf2ICpmWhWjDtDhwZ78PqYVFlp8Sknf+aORsL0lVaviVhd1EZ3hdkXiV4CXdGvM9etWoiQVAQg1nXf3sutHESBUAHCH7iKcP8gimBwpss4WLPFzEc39bTyCR3tUGDP4Hz9qlGnE2vWFa0lKkBV2FjC/0NHMN9Oybs1yj1pQnE7odi3I6YxVv7TU= 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: 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 --- 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 e16a4bc82856..660f9154a211 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(void) return is_power_of_2(sizeof(struct page)); } -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); -/* - * Return the real head page struct iff the @page is a fake head page, otherwise - * return the @page itself. See Documentation/mm/vmemmap_dedup.rst. - */ -static __always_inline const struct page *page_fixed_fake_head(const struct 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 = READ_ONCE(page[1].compound_info); - - /* See set_compound_head() */ - if (likely(info & 1)) { - unsigned long p = (unsigned long)page; - - return (const struct page *)(p & info); - } - } - return page; -} - -static __always_inline bool page_count_writable(const struct page *page, int 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) == u) - return false; - - return page_fixed_fake_head(page) == page; -} -#else -static inline const struct page *page_fixed_fake_head(const struct page *page) -{ - 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) != page; -} - static __always_inline unsigned long _compound_head(const struct page *page) { unsigned long info = READ_ONCE(page->compound_info); /* Bit 0 encodes PageTail() */ if (!(info & 1)) - return (unsigned long)page_fixed_fake_head(page); + return (unsigned long)page; /* * 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) 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; } static __always_inline int PageCompound(const struct page *page) @@ -924,7 +837,7 @@ static __always_inline bool folio_test_head(const struct 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); } __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 *folio) static inline bool page_ref_add_unless(struct page *page, int nr, int u) { - bool ret = false; - - rcu_read_lock(); - /* avoid writing to the vmemmap area being remapped */ - if (page_count_writable(page, u)) - ret = atomic_add_unless(&page->_refcount, nr, u); - rcu_read_unlock(); + bool ret = atomic_add_unless(&page->_refcount, nr, u); if (page_ref_tracepoint_active(page_ref_mod_unless)) __page_ref_mod_unless(page, nr, ret); -- 2.51.2