linux-kernel - Re: [PATCH 1/2] mm/debug_vm_pgtable: Use set

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Message-ID: <56560af0-6a94-43f7-ba9b-49ec476d1a1c@lucifer.local>
Date: Mon, 15 Dec 2025 10:33:54 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Samuel Holland <samuel.holland@...ive.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        David Hildenbrand <david@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Anshuman Khandual <anshuman.khandual@....com>,
        Gavin Shan <gshan@...hat.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH 1/2] mm/debug_vm_pgtable: Use set_pXd() to write page
 tables

On Thu, Dec 11, 2025 at 09:36:27AM +0000, Ryan Roberts wrote:
> On 11/12/2025 08:11, Samuel Holland wrote:
> > Generic code must always use the architecture-provided helper function
> > to write page tables.
> >
> > Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")
> > Fixes: c0fe07b0aa72 ("mm/debug_vm_pgtable: use struct pgtable_debug_args in PMD modifying tests")
> > Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> > ---
> >
> >  mm/debug_vm_pgtable.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> > index ae9b9310d96fd..8c7996d6c1f2a 100644
> > --- a/mm/debug_vm_pgtable.c
> > +++ b/mm/debug_vm_pgtable.c
> > @@ -445,7 +445,7 @@ static void __init pmd_huge_tests(struct pgtable_debug_args *args)
> >  	 * X86 defined pmd_set_huge() verifies that the given
> >  	 * PMD is not a populated non-leaf entry.
> >  	 */
> > -	WRITE_ONCE(*args->pmdp, __pmd(0));
> > +	set_pmd(args->pmdp, __pmd(0));
>
> My guess is that this should actually be:
>
> 	pmd_clear(args->pmdp);
>
> As the "clear" value may not be zero on some arches (see um) or there may be
> more to do on other arches (see arm32's 2 level pgtable).

Yeah, seems on looooongarch too for instance there's an invalid value being set
that is later checked by e.g. pmd_none().

>
> >  	WARN_ON(!pmd_set_huge(args->pmdp, __pfn_to_phys(args->fixed_pmd_pfn), args->page_prot));
> >  	WARN_ON(!pmd_clear_huge(args->pmdp));
> >  	pmd = pmdp_get(args->pmdp);
> > @@ -465,7 +465,7 @@ static void __init pud_huge_tests(struct pgtable_debug_args *args)
> >  	 * X86 defined pud_set_huge() verifies that the given
> >  	 * PUD is not a populated non-leaf entry.
> >  	 */
> > -	WRITE_ONCE(*args->pudp, __pud(0));
> > +	set_pud(args->pudp, __pud(0));
>
> Likewise, I think this should be:
>
> 	pud_clear(args->pudp);
>
> Thanks,
> Ryan
>
> >  	WARN_ON(!pud_set_huge(args->pudp, __pfn_to_phys(args->fixed_pud_pfn), args->page_prot));
> >  	WARN_ON(!pud_clear_huge(args->pudp));
> >  	pud = pudp_get(args->pudp);
>

Yeah I'm with Ryan on this, also as the kernel build bot attests, it turns out
we don't even define set_pmd()/set_pud() for powerpc... :)