[PATCH v2 1/1] mm: pgtable: fix pte_swp_exclusive

[Geert Uytterhoeven]

Hi Adrian,

On Sat, 5 Apr 2025 at 19:22, John Paul Adrian Glaubitz
<glaubitz at physik.fu-berlin.de> wrote:
> On Tue, 2025-02-18 at 18:55 +0100, Magnus Lindholm wrote:
> > Make pte_swp_exclusive return bool instead of int. This will better reflect
> > how pte_swp_exclusive is actually used in the code. This fixes swap/swapoff
> > problems on Alpha due pte_swp_exclusive not returning correct values when
> > _PAGE_SWP_EXCLUSIVE bit resides in upper 32-bits of PTE (like on alpha).
>
> Minor nitpick:
>
> "when _PAGE_SWP_EXCLUSIVE" => "when the _PAGE_SWP_EXCLUSIVE"
>
> >
> > Signed-off-by: Magnus Lindholm <linmag7 at gmail.com>

> > --- a/arch/alpha/include/asm/pgtable.h
> > +++ b/arch/alpha/include/asm/pgtable.h
> > @@ -334,7 +334,7 @@ extern inline pte_t mk_swap_pte(unsigned long type, unsigned long offset)
> >  #define __pte_to_swp_entry(pte)      ((swp_entry_t) { pte_val(pte) })
> >  #define __swp_entry_to_pte(x)        ((pte_t) { (x).val })
> >
> > -static inline int pte_swp_exclusive(pte_t pte)
> > +static inline bool pte_swp_exclusive(pte_t pte)
> >  {
> >       return pte_val(pte) & _PAGE_SWP_EXCLUSIVE;
> >  }

> > --- a/arch/xtensa/include/asm/pgtable.h
> > +++ b/arch/xtensa/include/asm/pgtable.h
> > @@ -355,7 +355,7 @@ ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
> >  #define __pte_to_swp_entry(pte)      ((swp_entry_t) { pte_val(pte) })
> >  #define __swp_entry_to_pte(x)        ((pte_t) { (x).val })
> >
> > -static inline int pte_swp_exclusive(pte_t pte)
> > +static inline bool pte_swp_exclusive(pte_t pte)
> >  {
> >       return pte_val(pte) & _PAGE_SWP_EXCLUSIVE;
> >  }
>
> I'm not so sure about this implicit cast from unsigned long to bool though.
>
> Is this verified to work correctly on all architectures? I wonder why this

Should work fine: any non-zero value is mapped to one.

> bug was not caught earlier on alpha on the other hand.

On Alpha, "pte_val(pte) & _PAGE_SWP_EXCLUSIVE" is either
_PAGE_SWP_EXCLUSIVE == 0x8000000000UL or zero.  Due to the return
type being int, the return value was truncated, and the function always
returned zero.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds