The patch titled Subject: mm/thp: fix MTE tag mismatch when replacing zero-filled subpages has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

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

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------------------------------------------------------ From: Lance Yang lance.yang@linux.dev Subject: mm/thp: fix MTE tag mismatch when replacing zero-filled subpages Date: Mon, 22 Sep 2025 10:14:58 +0800

When both THP and MTE are enabled, splitting a THP and replacing its zero-filled subpages with the shared zeropage can cause MTE tag mismatch faults in userspace.

Remapping zero-filled subpages to the shared zeropage is unsafe, as the zeropage has a fixed tag of zero, which may not match the tag expected by the userspace pointer.

KSM already avoids this problem by using memcmp_pages(), which on arm64 intentionally reports MTE-tagged pages as non-identical to prevent unsafe merging.

As suggested by David[1], this patch adopts the same pattern, replacing the memchr_inv() byte-level check with a call to pages_identical(). This leverages existing architecture-specific logic to determine if a page is truly identical to the shared zeropage.

Having both the THP shrinker and KSM rely on pages_identical() makes the design more future-proof, IMO. Instead of handling quirks in generic code, we just let the architecture decide what makes two pages identical.

[1] https://lore.kernel.org/all/ca2106a3-4bb2-4457-81af-301fd99fbef4@redhat.com

Link: https://lkml.kernel.org/r/20250922021458.68123-1-lance.yang@linux.dev Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Signed-off-by: Lance Yang lance.yang@linux.dev Reported-by: Qun-wei Lin Qun-wei.Lin@mediatek.com Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@m... Suggested-by: David Hildenbrand david@redhat.com Acked-by: Zi Yan ziy@nvidia.com Acked-by: David Hildenbrand david@redhat.com Acked-by: Usama Arif usamaarif642@gmail.com Reviewed-by: Catalin Marinas catalin.marinas@arm.com Cc: Alistair Popple apopple@nvidia.com Cc: andrew.yang andrew.yang@mediatek.com Cc: Baolin Wang baolin.wang@linux.alibaba.com Cc: Barry Song baohua@kernel.org Cc: Byungchul Park byungchul@sk.com Cc: Charlie Jenkins charlie@rivosinc.com Cc: Chinwen Chang chinwen.chang@mediatek.com Cc: Dev Jain dev.jain@arm.com Cc: Domenico Cerasuolo cerasuolodomenico@gmail.com Cc: Gregory Price gourry@gourry.net Cc: "Huang, Ying" ying.huang@linux.alibaba.com Cc: Hugh Dickins hughd@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Joshua Hahn joshua.hahnjy@gmail.com Cc: Kairui Song ryncsn@gmail.com Cc: Kalesh Singh kaleshsingh@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Mariano Pache npache@redhat.com Cc: Mathew Brost matthew.brost@intel.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Mike Rapoport rppt@kernel.org Cc: Palmer Dabbelt palmer@rivosinc.com Cc: Rakie Kim rakie.kim@sk.com Cc: Rik van Riel riel@surriel.com Cc: Roman Gushchin roman.gushchin@linux.dev Cc: Ryan Roberts ryan.roberts@arm.com Cc: Samuel Holland samuel.holland@sifive.com Cc: Shakeel Butt shakeel.butt@linux.dev Cc: Suren Baghdasaryan surenb@google.com Cc: Yu Zhao yuzhao@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/huge_memory.c | 15 +++------------ mm/migrate.c | 8 +------- 2 files changed, 4 insertions(+), 19 deletions(-)

--- a/mm/huge_memory.c~mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages +++ a/mm/huge_memory.c @@ -4115,32 +4115,23 @@ static unsigned long deferred_split_coun static bool thp_underused(struct folio *folio) { int num_zero_pages = 0, num_filled_pages = 0; - void *kaddr; int i;

if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) return false;

for (i = 0; i < folio_nr_pages(folio); i++) { - kaddr = kmap_local_folio(folio, i * PAGE_SIZE); - if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { - num_zero_pages++; - if (num_zero_pages > khugepaged_max_ptes_none) { - kunmap_local(kaddr); + if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { + if (++num_zero_pages > khugepaged_max_ptes_none) return true; - } } else { /* * Another path for early exit once the number * of non-zero filled pages exceeds threshold. */ - num_filled_pages++; - if (num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none) { - kunmap_local(kaddr); + if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none) return false; - } } - kunmap_local(kaddr); } return false; } --- a/mm/migrate.c~mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages +++ a/mm/migrate.c @@ -301,9 +301,7 @@ static bool try_to_map_unused_to_zeropag unsigned long idx) { struct page *page = folio_page(folio, idx); - bool contains_data; pte_t newpte; - void *addr;

if (PageCompound(page)) return false; @@ -320,11 +318,7 @@ static bool try_to_map_unused_to_zeropag * this subpage has been non present. If the subpage is only zero-filled * then map it to the shared zeropage. */ - addr = kmap_local_page(page); - contains_data = memchr_inv(addr, 0, PAGE_SIZE); - kunmap_local(addr); - - if (contains_data) + if (!pages_identical(page, ZERO_PAGE(0))) return false;

newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address), _

Patches currently in -mm which might be from lance.yang@linux.dev are

hung_task-fix-warnings-caused-by-unaligned-lock-pointers.patch mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages.patch selftests-mm-skip-soft-dirty-tests-when-config_mem_soft_dirty-is-disabled.patch