From: Lance Yang Date: Mon, 22 Sep 2025 02:14:58 +0000 (+0800) Subject: mm/thp: fix MTE tag mismatch when replacing zero-filled subpages X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=1ce6473d17e78e3cb9a40147658231731a551828;p=users%2Fhch%2Fmisc.git mm/thp: fix MTE tag mismatch when replacing zero-filled subpages 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 Reported-by: Qun-wei Lin Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@mediatek.com Suggested-by: David Hildenbrand Acked-by: Zi Yan Acked-by: David Hildenbrand Acked-by: Usama Arif Reviewed-by: Catalin Marinas Reviewed-by: Wei Yang Cc: Alistair Popple Cc: andrew.yang Cc: Baolin Wang Cc: Barry Song Cc: Byungchul Park Cc: Charlie Jenkins Cc: Chinwen Chang Cc: Dev Jain Cc: Domenico Cerasuolo Cc: Gregory Price Cc: "Huang, Ying" Cc: Hugh Dickins Cc: Johannes Weiner Cc: Joshua Hahn Cc: Kairui Song Cc: Kalesh Singh Cc: Liam Howlett Cc: Lorenzo Stoakes Cc: Mariano Pache Cc: Mathew Brost Cc: Matthew Wilcox (Oracle) Cc: Mike Rapoport Cc: Palmer Dabbelt Cc: Rakie Kim Cc: Rik van Riel Cc: Roman Gushchin Cc: Ryan Roberts Cc: Samuel Holland Cc: Shakeel Butt Cc: Suren Baghdasaryan Cc: Yu Zhao Cc: Signed-off-by: Andrew Morton --- diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 5acca24bbabb..1b81680b4225 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -4104,32 +4104,23 @@ static unsigned long deferred_split_count(struct shrinker *shrink, 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; } diff --git a/mm/migrate.c b/mm/migrate.c index aee61a980374..ce83c2c3c287 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -300,9 +300,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, unsigned long idx) { struct page *page = folio_page(folio, idx); - bool contains_data; pte_t newpte; - void *addr; if (PageCompound(page)) return false; @@ -319,11 +317,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, * 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),