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

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Message-ID: <791e0d59-0eb2-481f-bf8b-ba4b413d5ebd@redhat.com>
Date: Wed, 24 Sep 2025 11:44:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com, usamaarif642@...il.com, yuzhao@...gle.com,
 ziy@...dia.com, baolin.wang@...ux.alibaba.com, baohua@...nel.org,
 voidice@...il.com, Liam.Howlett@...cle.com, cerasuolodomenico@...il.com,
 hannes@...xchg.org, kaleshsingh@...gle.com, npache@...hat.com,
 riel@...riel.com, roman.gushchin@...ux.dev, rppt@...nel.org,
 ryan.roberts@....com, dev.jain@....com, ryncsn@...il.com,
 shakeel.butt@...ux.dev, surenb@...gle.com, hughd@...gle.com,
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 Andrew.Yang@...iatek.com, casper.li@...iatek.com,
 chinwen.chang@...iatek.com, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing
 zero-filled subpages

On 24.09.25 11:34, Catalin Marinas wrote:
> 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().
> 

This one:

commit d82d09e482199e6bbc204df10b2082f764cbe1f4
Author: David Hildenbrand <david@...hat.com>
Date:   Mon Aug 11 13:26:25 2025 +0200

     mm/huge_memory: mark PMD mappings of the huge zero folio special

     The huge zero folio is refcounted (+mapcounted -- is that a word?)
     differently than "normal" folios, similarly (but different) to the
     ordinary shared zeropage.


It should be in mm-stable, to go upstream in the upcoming merge window. 
It's been lurking in -next for a while now.

As it behaves just like the ordinary shared zeropage now, would we have 
to zero/initialize the tags after allocating it?

-- 
Cheers

David / dhildenb