Re: [PATCH mm-new 2/3] mm: clean up and expose is_guard_pte_marker()

[Lance Yang]

Hi Lorenzo,

Thanks for taking time to review!

On 2025/9/15 21:54, Lorenzo Stoakes wrote:
> On Sun, Sep 14, 2025 at 10:35:46PM +0800, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > is_guard_pte_marker() performs a redundant check because it calls both
> > is_pte_marker() and is_guard_swp_entry(), both of which internally check
> > for a PTE marker.
> >
> > is_guard_pte_marker()
> >   |- is_pte_marker()
> >   |   `- is_pte_marker_entry()  // First check
> >   `- is_guard_swp_entry()
> >       `- is_pte_marker_entry()  // Second, redundant check
>
> I mean, it expands to:
>
> is_swap_pte(pte) && is_pte_marker_entry(pte_to_swp_entry(pte)) &&
> is_pte_marker_entry(pte_to_swp_entry(pte))

Yes, that's a much clearer way to lay it out ;)

> So I don't think it's really unreasonable to expect compiler magic here...
>
> But you're right that I should have just used is_swap_pte() really, it's a bit
> silly not to, so this is fine :)

Exactly. Glad we're on the same page!

> > While a modern compiler could likely optimize this away, let's have clean
> > code and not rely on it ;)
>
> Please don't put smileys in commit messages :) as cute as they are, this is
> going on the permanent kernel record and while we all love them, it's
> probably not the best place to put them :P
>
> > Also, make it available for hugepage collapsing code.
>
> Nit but put a newline after this.

Got it. Will fix up all nits in v2.

> I think probably if I'm really really nitty I'd say that you should put
> this bit first, as it's the primary motivation for the change, and put the
> refactoring stuff after.

Ah, right. The motivation for exposing the helper should come first. I'll
reorder this changelog in v2.

> > Cc: Kairui Song <kasong@xxxxxxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
>
> This seems fine to me, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Thanks,
Lance

> > ---
> >   include/linux/swapops.h | 6 ++++++
> >   mm/madvise.c            | 6 ------
> >   2 files changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index 59c5889a4d54..7f5684fa043b 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -469,6 +469,12 @@ static inline int is_guard_swp_entry(swp_entry_t entry)
> >   		(pte_marker_get(entry) & PTE_MARKER_GUARD);
> >   }
> >
> > +static inline bool is_guard_pte_marker(pte_t ptent)
> > +{
> > +	return is_swap_pte(ptent) &&
> > +	       is_guard_swp_entry(pte_to_swp_entry(ptent));
> > +}
> > +
> >   /*
> >    * This is a special version to check pte_none() just to cover the case when
> >    * the pte is a pte marker.  It existed because in many cases the pte marker
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 35ed4ab0d7c5..bd46e6788fac 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1069,12 +1069,6 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
> >   	return !(vma->vm_flags & disallowed);
> >   }
> >
> > -static bool is_guard_pte_marker(pte_t ptent)
> > -{
> > -	return is_pte_marker(ptent) &&
> > -		is_guard_swp_entry(pte_to_swp_entry(ptent));
> > -}
> > -
> >   static int guard_install_pud_entry(pud_t *pud, unsigned long addr,
> >   				   unsigned long next, struct mm_walk *walk)
> >   {
> > --
> > 2.49.0