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Message-ID: <e4e82695-c03f-4105-bddd-9778d7e368d4@linux.dev>
Date: Mon, 22 Sep 2025 11:36:11 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Zi Yan <ziy@...dia.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
usamaarif642@...il.com, yuzhao@...gle.com, baolin.wang@...ux.alibaba.com,
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apopple@...dia.com, qun-wei.lin@...iatek.com, Andrew.Yang@...iatek.com,
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linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mediatek@...ts.infradead.org, linux-mm@...ck.org, ioworker0@...il.com,
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palmer@...osinc.com, samuel.holland@...ive.com, charlie@...osinc.com
Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing
zero-filled subpages
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@...ux.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@...r.kernel.org>
>> Reported-by: Qun-wei Lin <Qun-wei.Lin@...iatek.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@...hat.com>
>> Signed-off-by: Lance Yang <lance.yang@...ux.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@...dia.com>
Cheers!
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