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Fri, 5 Dec 2025 14:44:09 -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 10/11] mm: Remove the branch from compound_head() Date: Fri, 5 Dec 2025 19:43:46 +0000 Message-ID: <20251205194351.1646318-11-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: krfqgedgu7dk8dk9xdu7sz87jjaqxx5b X-Rspamd-Queue-Id: 9479D4000C X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1764963851-296721 X-HE-Meta: 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 NXzOshPv o0mi0PQozaW24wYXFdMRs/F5ASeEyZWZLwrxY3w+p2fIANAlB6Pe+TLTMeL4b2fQ/J3xPL64HYPikQDBvvKbJKOCfCU5FD6hglNWkSiEFovmxfzdkQDNjbmRd1AYeSt07pOrxFy9vRrDKjNzabWsfJUFmQaXkc3/Qthv3Qvs4/89jo8e2fAqX5MKE651KD5h9rFIuV5W1PqO9SSDWaFD664QRriOHLiSck84OcByhBncjPnG+lxSVp4S3WRc80fcwcobF9h07DYV2WKXclQv53oO5c7FRYN6hnWIMjI/qX7f5x9uk0gT0do0XSQ== 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 --- 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 02a851ab7f5e..01d9893c4bd8 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -201,17 +201,15 @@ enum pageflags { static __always_inline unsigned long _compound_head(const struct page *page) { unsigned long info = READ_ONCE(page->compound_info); + unsigned long mask; + + if (!is_power_of_2(sizeof(struct page))) { + /* Bit 0 encodes PageTail() */ + if (info & 1) + return info - 1; - /* Bit 0 encodes PageTail() */ - if (!(info & 1)) return (unsigned long)page; - - /* - * If the size of struct page is not power-of-2, the rest if - * compound_info is the pointer to the head page. - */ - if (!is_power_of_2(sizeof(struct page))) - return info - 1; + } /* * If the size of struct page is power-of-2 it is set the rest of @@ -219,8 +217,17 @@ static __always_inline unsigned long _compound_head(const struct page *page) * 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