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Mon, 2 Feb 2026 10:58:42 -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 15/17] mm: Remove the branch from compound_head() Date: Mon, 2 Feb 2026 15:56:31 +0000 Message-ID: <20260202155634.650837-16-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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: F104D1C0012 X-Stat-Signature: g8wqj796ytox8r8bkmoqor3n5p6z8gid X-Rspam-User: X-HE-Tag: 1770047926-424440 X-HE-Meta: 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 9Tzth8lF SWzF8mnptrYfHlflepzpHRzi0t5iwKpYKX7cxa6WUDThMdCZ/1Okph/Lu24/BtuBIQWXZKkp1q/5/Wqq/uqVlbHTtvYK2Sm8zWehv3RIbex/Ht8Yk3+MGKvqS3oXVWvspEKex/emYrOf22DtktrbysatpIak7V849wbL/gKyEQ5b0RX3Vcra5tb0jLdIvc1uLt5V27ldfBLE5d5GUonqYN3hW7rsTdp+c65frionyPgEe0KksnfKJJdUbqgH1O0h7UWlcwZlJOZP3v0IfxqRtt8572eXadlFsWBW57Tf45gyS8pxo0S3A3MhIsvrX88pp2M/vgjDXXejzQB0= 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: 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 --- 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 1aaa604f4b9b..16384cb6f962 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 compoun_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