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

[David Hildenbrand <david@redhat.com>]

On 24.09.25 10:50, Catalin Marinas wrote:
> On Wed, Sep 24, 2025 at 10:49:27AM +0800, Lance Yang wrote:
>> On 2025/9/24 00:14, Catalin Marinas wrote:
>>> So alternative patch that also fixes the deferred struct page init (on
>>> the assumptions that the zero page is always mapped as pte_special():
>>
>> I can confirm that this alternative patch also works correctly; my tests
>> for MTE all pass ;)
> 
> Thanks Lance for testing. I'll post one of the variants today.
> 
>> This looks like a better fix since it solves the boot hang issue too.
> 
> In principle, yes, until I tracked down why I changed it in the first
> place - 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to
> the zero page"). ptrace() can read tags from PROT_MTE mappings and we
> want to allow reading zeroes as well if the page points to the zero
> page. Not flagging the page as PG_mte_tagged caused issues.
> 
> I can change the logic in the ptrace() code, I just need to figure out
> what happens to the huge zero page. Ideally we should treat both in the
> same way but, AFAICT, we don't use pmd_mkspecial() on the huge zero
> page, so it gets flagged with PG_mte_tagged.

I changed that recently :) The huge zero folio will now always have 
pmd_special() set.

-- 
Cheers

David / dhildenb