Re: [PATCH] mm/ptdump: Replace READ_ONCE() with standard page table accessors

[Dev Jain]

On 30/09/25 8:13 pm, David Hildenbrand wrote:
> On 30.09.25 09:00, Anshuman Khandual wrote:
> > On 30/09/25 12:11 PM, David Hildenbrand wrote:
> > > On 30.09.25 06:37, Dev Jain wrote:
> > > > On 30/09/25 8:22 am, Anshuman Khandual wrote:
> > > > > Replace READ_ONCE() with standard page table accessors i.e 
> > > > > pxdp_get() which
> > > > > anyways default into READ_ONCE() in cases where platform does not 
> > > > > override.
> > > > >
> > > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > > > > Cc: linux-mm@xxxxxxxxx
> > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > > > ---
> > > > >     mm/ptdump.c | 8 ++++----
> > > > >     1 file changed, 4 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/mm/ptdump.c b/mm/ptdump.c
> > > > > index b600c7f864b8..18861501b533 100644
> > > > > --- a/mm/ptdump.c
> > > > > +++ b/mm/ptdump.c
> > > > > @@ -31,7 +31,7 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned 
> > > > > long addr,
> > > > >                     unsigned long next, struct mm_walk *walk)
> > > > >     {
> > > > >         struct ptdump_state *st = walk->private;
> > > > > -    pgd_t val = READ_ONCE(*pgd);
> > > > > +    pgd_t val = pgdp_get(pgd);
> > > > >        #if CONFIG_PGTABLE_LEVELS > 4 && \
> > > > >             (defined(CONFIG_KASAN_GENERIC) || 
> > > > > defined(CONFIG_KASAN_SW_TAGS))
> > > > > @@ -54,7 +54,7 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned 
> > > > > long addr,
> > > > >                     unsigned long next, struct mm_walk *walk)
> > > > >     {
> > > > >         struct ptdump_state *st = walk->private;
> > > > > -    p4d_t val = READ_ONCE(*p4d);
> > > > > +    p4d_t val = p4dp_get(p4d);
> > > > >        #if CONFIG_PGTABLE_LEVELS > 3 && \
> > > > >             (defined(CONFIG_KASAN_GENERIC) || 
> > > > > defined(CONFIG_KASAN_SW_TAGS))
> > > > > @@ -77,7 +77,7 @@ static int ptdump_pud_entry(pud_t *pud, unsigned 
> > > > > long addr,
> > > > >                     unsigned long next, struct mm_walk *walk)
> > > > >     {
> > > > >         struct ptdump_state *st = walk->private;
> > > > > -    pud_t val = READ_ONCE(*pud);
> > > > > +    pud_t val = pudp_get(pud);
> > > > >        #if CONFIG_PGTABLE_LEVELS > 2 && \
> > > > >             (defined(CONFIG_KASAN_GENERIC) || 
> > > > > defined(CONFIG_KASAN_SW_TAGS))
> > > > > @@ -100,7 +100,7 @@ static int ptdump_pmd_entry(pmd_t *pmd, 
> > > > > unsigned long addr,
> > > > >                     unsigned long next, struct mm_walk *walk)
> > > > >     {
> > > > >         struct ptdump_state *st = walk->private;
> > > > > -    pmd_t val = READ_ONCE(*pmd);
> > > > > +    pmd_t val = pmdp_get(pmd);
> > > >
> > > > I believe this should go through pmdp_get_lockless(). I can see in 
> > > > pgtable.h that
> > > > some magic is required on some arches to decode the pmd correctly 
> > > > in case walking
> > > > without locks.
> > >
> > > pmdp_get_lockless() is a nasty thingy to handle selected 32bit 
> > > architectures.
> > >
> > > But given that we're using ptep_get_lockless() in ptdump_pmd_entry() 
> > > it probably wouldn't hurt to use pmdp_get_lockless() here.
> > >
> > > Staring at ARCH_HAS_PTDUMP, I don't think any 32bit arch would 
> > > actually end up compiling ptdump.c.
> > >
> > > E.g., on x86 only X86_64 ends up selecting ARCH_HAS_PTDUMP.
> >
> > pxdp_get_lockless() not really required here, let's stick with 
> > pxdp_get() instead.
>
> I'd suggest that we keep it consistent. That is, also removing the 
> ptep_get_lockless() if not really required.

So in theory, the _lockless variant is not required, but surely 
consistency would mean that since we are

dereferencing the pte without the PTL, we should use the _lockless variant?