Re: [PATCH v2 6/6] mm: constify the pte_offset_map_ro_nolock() return value

[Pedro Falcato <pfalcato@suse.de>]

On Wed, Aug 05, 2026 at 07:58:02AM +0200, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 04/08/2026 à 21:22, Pedro Falcato a écrit :
> > On Tue, Aug 04, 2026 at 12:22:19PM +0100, Lorenzo Stoakes (ARM) wrote:
> > 
> > > 
> > > >   			if (!pte) {
> > > >   				mmap_read_unlock(mm);
> > > >   				result = SCAN_NO_PTE_TABLE;
> > > > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> > > > index b91b1a98029c..2cfc6e608ef4 100644
> > > > --- a/mm/pgtable-generic.c
> > > > +++ b/mm/pgtable-generic.c
> > > > @@ -308,8 +308,8 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
> > > >   	return NULL;
> > > >   }
> > > > 
> > > > -pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> > > > -				unsigned long addr, spinlock_t **ptlp)
> > > > +const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> > > 
> > > Can pmd be const too?
> > 
> > I was going to say "yes but then pmdp_get() also needs to be constfified" but
> > actually no, it can't:
> > 
> > pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp) {
> >          if (unlikely(pmd_bad(pmdval))) {
> >                  pmd_clear_bad(pmd);
> >                  goto nomap;
> >          }
> > }
> > 
> > so PTE mapping actually needs to write to the pmdp if the pmd looks bad.
> > Tricky stuff :)
> 
> But if pmd is const, can it be bad at all ?

Yes, you just need a stray write or a bit of memory corruption and it can
go bad. And then we need to do clear_bad() :)

(it's unclear to me whether this is actually common or useful enough these
days; the way this was explained to me, page tables can be best-effort
redundant; but it's not like we know clearing the whole range is ok, and
the way pmd_ERROR, etc work they don't even communicate to userspace what
happened, unlike normal hwpoison mechanisms)

-- 
Pedro