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Wed, 21 Jan 2026 11:23:22 -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: [PATCHv4 13/14] mm: Remove the branch from compound_head() Date: Wed, 21 Jan 2026 16:22:50 +0000 Message-ID: <20260121162253.2216580-14-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4A4E120003 X-Stat-Signature: yzzh3347d4g1h7s4dakagte814zm8e4f X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1769012605-178526 X-HE-Meta: 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 8NCooqaP 6kCbKE6MWqcsUXFEkaE6kZNyFUo8lHjot/1o4fwAgxLtX/4sGcNWLo8eooBOo06Dfg52+84VA+Zw0Pv2mlhnVtlGFNFesyvZYLjZ/UVXLCgDUALnxfCSVp7LSpVrQzMpnu32Hz4b/iXhv1zJ1xczO1lhg8Lu1c29375BqLrTs7aqYXIofEqcwUBOiCS5k6eQVFmrcG8w1b+6kZ6za43KGuLuP75oGpNSmOqXYWi/Z9ZMHYuQesaIk6+GdSmShrdnLO5mAE6ogZpAmCY0V331+FDsat3citIia31gBzrkiauLB66M1R46QNGtkqA== 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