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

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Sep 24, 2025 at 11:44:19AM +0200, David Hildenbrand wrote:
> 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@redhat.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.

Thanks. At least it's something to address in the next kernel version. I
need to improve the MTE kselftests to catch the zero page scenarios.

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

Yes. Before pmd_special(), it was be done lazily via set_pmd_at(). I
think it just needs a __GFP_ZEROTAGS. The only other place we use this
flag is in vma_alloc_zeroed_movable_folio(), as an optimisation to avoid
a separate loop for zeroing the tags after data.

-- 
Catalin