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

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Message-ID: <aNOwuKmbAaMaEMb7@arm.com>
Date: Wed, 24 Sep 2025 09:50:00 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing
 zero-filled subpages

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.

If I go with the first fix for the page merging, I'll look to defer the
zero page initialisation post page_alloc_init_late() in a separate
patch.

-- 
Catalin