+ powerpc-mm-map-shared-vmemmap-tail-pages-as-read-only.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: powerpc/mm: map shared vmemmap tail pages as read-only
has been added to the -mm mm-new branch.  Its filename is
     powerpc-mm-map-shared-vmemmap-tail-pages-as-read-only.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/powerpc-mm-map-shared-vmemmap-tail-pages-as-read-only.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

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Muchun Song <muchun.song@linux.dev>
Subject: powerpc/mm: map shared vmemmap tail pages as read-only
Date: Mon, 19 Oct 2026 17:08:28 +0800

PowerPC radix uses an architecture-specific vmemmap population path for
shared tail mappings, so it does not inherit the read-only protection added
to the common path.

Protect these mappings as well so illegal writes fault instead of
corrupting metadata shared by multiple tail pages.

Link: https://lore.kernel.org/20261019090830.1917657-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
---

 arch/powerpc/mm/book3s64/radix_pgtable.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/powerpc/mm/book3s64/radix_pgtable.c~powerpc-mm-map-shared-vmemmap-tail-pages-as-read-only
+++ a/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -1059,6 +1059,9 @@ static pte_t * __meminit radix__vmemmap_
 
 		VM_BUG_ON(!PAGE_ALIGNED(addr));
 		entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL);
+		/* Map the shared tail page as read-only to catch illegal writes. */
+		if (reuse)
+			entry = pte_wrprotect(entry);
 		set_pte_at(&init_mm, addr, pte, entry);
 		asm volatile("ptesync": : :"memory");
 	}
_

Patches currently in -mm which might be from muchun.song@linux.dev are

riscv-mm-fix-hotplug-page-table-destructor-handling.patch
riscv-mm-fix-missing-destructor-for-hotplug-pud-tables.patch
mm-fix-missing-page-table-destructor-in-pagetable_free.patch
arm64-mm-fix-destructor-for-unconstructed-hotplug-page-tables.patch
mm-sparse-vmemmap-map-shared-vmemmap-tail-pages-as-read-only.patch
powerpc-mm-map-shared-vmemmap-tail-pages-as-read-only.patch
mm-hugetlb_vmemmap-remove-vmemmap_wrprotect_hvo.patch
mm-hugetlb_vmemmap-remove-hugetlb_vmemmap_optimize_bootmem_folios.patch