Messages in this thread |  | | From | Zi Yan <> | | Subject | Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages | | Date | Sun, 21 Sep 2025 22:36:31 -0400 |
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On 21 Sep 2025, at 22:14, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev> > > 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 > > Cc: <stable@vger.kernel.org> > Reported-by: Qun-wei Lin <Qun-wei.Lin@mediatek.com> > Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@mediatek.com > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") > Suggested-by: David Hildenbrand <david@redhat.com> > Signed-off-by: Lance Yang <lance.yang@linux.dev> > --- > Tested on x86_64 and on QEMU for arm64 (with and without MTE support), > and the fix works as expected.
From [1], I see you mentioned RISC-V also has the address masking feature. Is it affected by this? And memcmp_pages() is only implemented by ARM64 for MTE. Should any arch with address masking always implement it to avoid the same issue?
> > mm/huge_memory.c | 15 +++------------ > mm/migrate.c | 8 +------- > 2 files changed, 4 insertions(+), 19 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 32e0ec2dde36..28d4b02a1aa5 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4104,29 +4104,20 @@ 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; > > 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), > -- > 2.49.0
The changes look good to me. Thanks. Acked-by: Zi Yan <ziy@nvidia.com>
-- Best Regards, Yan, Zi
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