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Thu, 15 Jan 2026 09:46:30 -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 14/15] mm: Remove the branch from compound_head() Date: Thu, 15 Jan 2026 14:46:00 +0000 Message-ID: <20260115144604.822702-15-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-Rspamd-Queue-Id: 6D183180009 X-Rspamd-Server: rspam06 X-Stat-Signature: q4f6r38buh5o6qd1ucthdyj8mu1q68yh X-Rspam-User: X-HE-Tag: 1768488393-855828 X-HE-Meta: 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 25rtwTWs JPVcas226a7J8YZrmsD7astMqkR0L3ovsC7tD9eiapQPMjmqNlHNqHAUuTwHU+gJrQ57Zr0TNy3zBcfQsBHScEeqrPiqSggcKaDbBVebcrvp2sWnpV6OM3rakpo/MQND/gQVvpWfsgKkP2EoaSEMeH0snNWmpIg19AJtyBmmlAAGr9IGmL7UHr+dK+TuQ3Yh1dzJW3ZJX6XGB6FQlvgMbqZOeE3Rb0JS1erzX2edzplfhJQVaF8TmUMeSPymZCd08jG30YI1tWhT0r2j1NHyyTZE0NdJ72yYSc7gi3I7MDi7oYtM60IrpQ70Aew== 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 --- 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 f89702e101e8..95ac963b78af 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