linux-kernel - Re: [PATCH] mm/dirty: Replace READ_ONCE() with pudp

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Message-ID: <CABzRoya4kGWCFL95B2NEoLk4Qk9c3Vs1694NSWPFhiQiFHyomg@mail.gmail.com>
Date: Mon, 6 Oct 2025 14:48:35 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Dev Jain <dev.jain@....com>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/dirty: Replace READ_ONCE() with pudp_get()

On Mon, Oct 6, 2025 at 2:28 PM Anshuman Khandual
<anshuman.khandual@....com> wrote:
>
>
>
> On 06/10/25 11:37 AM, Dev Jain wrote:
> >
> > On 06/10/25 11:22 am, Anshuman Khandual wrote:
> >> Replace READ_ONCE() with a standard page table accessor i.e pudp_get() that
> >> anyways defaults into READ_ONCE() in cases where platform does not override

Nice cleanup!

> >>
> >> Cc: Andrew Morton <akpm@...ux-foundation.org>
> >> Cc: David Hildenbrand <david@...hat.com>
> >> Cc: linux-mm@...ck.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> >> ---
> >>   mm/mapping_dirty_helpers.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/mapping_dirty_helpers.c b/mm/mapping_dirty_helpers.c
> >> index c193de6cb23a..737c407f4081 100644
> >> --- a/mm/mapping_dirty_helpers.c
> >> +++ b/mm/mapping_dirty_helpers.c
> >> @@ -149,7 +149,7 @@ static int wp_clean_pud_entry(pud_t *pud, unsigned long addr, unsigned long end,
> >>                     struct mm_walk *walk)
> >>   {
> >>   #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> >> -    pud_t pudval = READ_ONCE(*pud);
> >> +    pud_t pudval = pudp_get(pud);
> >>         /* Do not split a huge pud */
> >>       if (pud_trans_huge(pudval)) {
> >
> > Talking about mm, why not also make changes for these READ_ONCE accesses
> > in gup, hmm, memory, mprotect, sparse-vmemmap?

Yep, I agree with Dev on that one. Because the change is assumed to be a
no-op on all architectures for now, right?

> >
>
> Right, could replace all mm/ READ_ONCE() for pxdp pointers with the pgtable helpers
> but that will create too much code churn in a single patch. Thought of doing these
> replacements per file will be much more contained which is easy both for review and
> testing.

Emm... as pointed out by Dev, it would get the entire cleanup done in one
go, avoiding the churn of multiple small patches ;)

Cheers,
Lance