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Fri, 27 Feb 2026 14:43:35 -0500 (EST) From: "Kiryl Shutsemau (Meta)" 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: [PATCHv7 16/18] mm: Remove the branch from compound_head() Date: Fri, 27 Feb 2026 19:42:54 +0000 Message-ID: <20260227194302.274384-17-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260227194302.274384-1-kas@kernel.org> References: <20260227194302.274384-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: A246B1C000A X-Stat-Signature: gyp6b41smh3qi3sbfdma1ahrsq9mwqbz X-Rspam-User: X-HE-Tag: 1772221417-478272 X-HE-Meta: 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 PnOAAWD0 v4JkfxlYjAtk5KmlA0yCTAdZgc7edf5H44b63q5Z6idxfKesuiXgj41800AAI2ilyo7Z4ICcE9LZxtjETxHchQRUAuvJ+Yf8haAjjaqbNlz2Vv3ACDM7KRpeb15eQcK/rK4kaFICR87eS77jfmeVEbSFZwi80wX39FCV2N8EwVUX/cBSq95j+MXXTYuwrGlRTEzrwDwbdPOX8ZijZkQ1Bgkm0Abg59HZPFCyYdTygoyg4b7fZlM9SvlKbRoa356+g73m4kyBeRSlhHLT57+UtH36udyMa6mtQw9G0edO9MMyB2jx5Yn4H580xDiQHqyJKoJUtT+pFT3m8KCw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Kiryl Shutsemau The compound_head() function is a hot path. For example, the zap path calls it for every leaf page table entry. Rewrite the helper function in a branchless manner to eliminate the risk of CPU branch misprediction. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan Acked-by: David Hildenbrand (Arm) Reviewed-by: Vlastimil Babka --- include/linux/page-flags.h | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 8716a27ef347..9ef2c3b28e07 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void) static __always_inline unsigned long _compound_head(const struct page *page) { unsigned long info = READ_ONCE(page->compound_info); + unsigned long mask; + + if (!compound_info_has_mask()) { + /* Bit 0 encodes PageTail() */ + if (info & 1) + return info - 1; - /* Bit 0 encodes PageTail() */ - if (!(info & 1)) return (unsigned long)page; - - /* - * If compound_info_has_mask() is false, the rest of compound_info is - * the pointer to the head page. - */ - if (!compound_info_has_mask()) - return info - 1; + } /* * If compound_info_has_mask() is true the rest of the info encodes * the mask that converts the address of the tail page to the head page. * * No need to clear bit 0 in the mask as 'page' always has it clear. + * + * Let's do it in a branchless manner. */ - return (unsigned long)page & info; + + /* Non-tail: -1UL, Tail: 0 */ + mask = (info & 1) - 1; + + /* Non-tail: -1UL, Tail: info */ + mask |= info; + + return (unsigned long)page & mask; } #define compound_head(page) ((typeof(page))_compound_head(page)) -- 2.51.2