Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages

[David Hildenbrand <david@redhat.com>]

On 22.09.25 19:24, Catalin Marinas wrote:
> On Mon, Sep 22, 2025 at 10:14:58AM +0800, 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>
> 
> Functionally, the patch looks fine, both with and without MTE.
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> 
>> 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;
> 
> I wonder what the overhead of doing a memcmp() vs memchr_inv() is. The
> former will need to read from two places. If it's noticeable, it would
> affect architectures that don't have an MTE equivalent.
> 
> Alternatively we could introduce something like folio_has_metadata()
> which on arm64 simply checks PG_mte_tagged.

We discussed something similar in the other thread (I suggested 
page_is_mergable()). I'd prefer to use pages_identical() for now, so we 
have the same logic here and in ksm code.

(this patch here almost looks like a cleanup :) )

If this becomes a problem, what we could do is in pages_identical() 
would be simply doing the memchr_inv() in case is_zero_pfn(). KSM might 
benefit from that as well when merging with the shared zeropage through 
try_to_merge_with_zero_page().

-- 
Cheers

David / dhildenb