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

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Message-ID: <aNO7MrQt9oPT8Hic@arm.com>
Date: Wed, 24 Sep 2025 10:34:42 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: David Hildenbrand <david@...hat.com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com, usamaarif642@...il.com,
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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 11:13:18AM +0200, David Hildenbrand wrote:
> 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.

Oh, which commit was this? It means that we can end up with
uninitialised tags if we have a PROT_MTE huge zero page since
set_pmd_at/set_pte_at() skips mte_sync_tags().

-- 
Catalin