+ mm-remove-the-branch-from-compound_head.patch added to mm-new branch

+ mm-remove-the-branch-from-compound_head.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: remove the branch from compound_head()
has been added to the -mm mm-new branch.  Its filename is
     mm-remove-the-branch-from-compound_head.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-remove-the-branch-from-compound_head.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

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------------------------------------------------------
From: Kiryl Shutsemau <kas@kernel.org>
Subject: mm: remove the branch from compound_head()
Date: Fri, 27 Feb 2026 19:42:54 +0000

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.

Link: https://lkml.kernel.org/r/20260227194302.274384-17-kas@kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/page-flags.h |   27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

--- a/include/linux/page-flags.h~mm-remove-the-branch-from-compound_head
+++ a/include/linux/page-flags.h
@@ -229,25 +229,32 @@ static __always_inline int page_is_fake_
 static __always_inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long info = READ_ONCE(page->compound_info);
+	unsigned long mask;
 
-	/* Bit 0 encodes PageTail() */
-	if (!(info & 1))
-		return (unsigned long)page;
+	if (!compound_info_has_mask()) {
+		/* Bit 0 encodes PageTail() */
+		if (info & 1)
+			return info - 1;
 
-	/*
-	 * 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;
+		return (unsigned long)page;
+	}
 
 	/*
 	 * 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))
_

Patches currently in -mm which might be from kas@kernel.org are

mm-move-max_folio_order-definition-to-mmzoneh.patch
mm-change-the-interface-of-prep_compound_tail.patch
mm-rename-the-compound_head-field-in-the-struct-page-to-compound_info.patch
mm-move-set-clear_compound_head-next-to-compound_head.patch
riscv-mm-align-vmemmap-to-maximal-folio-size.patch
loongarch-mm-align-vmemmap-to-maximal-folio-size.patch
mm-rework-compound_head-for-power-of-2-sizeofstruct-page.patch
mm-sparse-check-memmap-alignment-for-compound_info_has_mask.patch
mm-hugetlb-defer-vmemmap-population-for-bootmem-hugepages.patch
mm-hugetlb-refactor-code-around-vmemmap_walk.patch
x86-vdso-undefine-config_hugetlb_page_optimize_vmemmap-for-vdso32.patch
mm-hugetlb-remove-fake-head-pages.patch
mm-drop-fake-head-checks.patch
hugetlb-remove-vmemmap_synchronize_rcu.patch
mm-hugetlb-remove-hugetlb_optimize_vmemmap_key-static-key.patch
mm-remove-the-branch-from-compound_head.patch
hugetlb-update-vmemmap_deduprst.patch
mm-slab-use-compound_head-in-page_slab.patch