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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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: pbeq766h8upiu76qpa9dr6n9jj6uj1r9 X-Rspam-User: X-Rspamd-Queue-Id: BBA521C0016 X-Rspamd-Server: rspam01 X-HE-Tag: 1764963847-438982 X-HE-Meta: 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 uALPdH4Z fhNP3Dq2obe8btAN232hWg7LaSut541OA0qIlOq/z7e0Anh5tKNdoNbVTlrz3HkGviEnaoH7BhYSwTsps2uClQQHnfYccfSATTNPrw7wRC8mef0pWOMrZrqM4x/D2rqfkbiy+5TIqGVBvShJgfFoSO7yYRINpZCSWF+MBbvqU7TfggeFCT2sog8bW/+BUgvHhjulc1Bf2zrVc2lvaY8N1o+JS0w5U853Cj76V0EZh51r5AYCI4b3p8dkiOzbPmpbGHq1XnnWm2HMotBg= 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: 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 { #ifndef __GENERATING_BOUNDS_H -#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 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 = READ_ONCE(page[1].compound_info); - - 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 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) 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) @@ -904,7 +815,7 @@ 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