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

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Message-ID: <17dabd83-0849-44c9-b4a2-196af60d9676@redhat.com>
Date: Wed, 24 Sep 2025 11:13:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Catalin Marinas <catalin.marinas@....com>,
 Lance Yang <lance.yang@...ux.dev>
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Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing
 zero-filled subpages

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