Messages in this thread Patch in this message |  | | Date | Mon, 22 Sep 2025 11:36:11 +0800 | | Subject | Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages | | From | Lance Yang <> |
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Cc: RISC-V folks
On 2025/9/22 10:36, Zi Yan wrote: > 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?
Yeah, I'm new to RISC-V, seems like RISC-V has a similar feature as described in Documentation/arch/riscv/uabi.rst, which is the Supm (Supervisor-mode Pointer Masking) extension.
``` Pointer masking ---------------
Support for pointer masking in userspace (the Supm extension) is provided via the ``PR_SET_TAGGED_ADDR_CTRL`` and ``PR_GET_TAGGED_ADDR_CTRL`` ``prctl()`` operations. Pointer masking is disabled by default. To enable it, userspace must call ``PR_SET_TAGGED_ADDR_CTRL`` with the ``PR_PMLEN`` field set to the number of mask/tag bits needed by the application. ``PR_PMLEN`` is interpreted as a lower bound; if the kernel is unable to satisfy the request, the ``PR_SET_TAGGED_ADDR_CTRL`` operation will fail. The actual number of tag bits is returned in ``PR_PMLEN`` by the ``PR_GET_TAGGED_ADDR_CTRL`` operation. ```
But, IIUC, Supm by itself only ensures that the upper bits are ignored on memory access :)
So, RISC-V today would likely not be affected. However, once it implements full hardware tag checking, it will face the exact same zero-page problem.
Anyway, any architecture with a feature like MTE in the future will need its own memcmp_pages() to prevent unsafe merges ;)
> >> >> 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>
Cheers!
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