On Tue, Sep 10, 2024 at 10:08:10AM +0100, Ryan Roberts wrote:
> On 10/09/2024 10:04, Anshuman Khandual wrote:
> > All set_pte_safe() usage have been dropped after the commit eccd906484d1
> > ("x86/mm: Do not use set_{pud, pmd}_safe() when splitting a large page")
> > This just drops now unused helper set_pte_safe().
>
> It would be good to add some comment here to mention that the macro was buggy
> due to doing direct dereferencing of the pte, and that if it were to be kept, it
> should have been updated to use a single call to ptep_get().
I'm not sure that the _macro_ would have been buggy in such a scenario.
If I understand correctly, the _caller_ would have been buggy:
/*
* Use set_p*_safe(), and elide TLB flushing, when confident that *no*
* TLB flush will be required as a result of the "set". For example, use
* in scenarios where it is known ahead of time that the routine is
* setting non-present entries, or re-setting an existing entry to the
* same value. Otherwise, use the typical "set" helpers and flush the
* TLB.
*/
so if *ptep changes between the two calls, that's the caller's bug,
right?
Otherwise, set_pmd_safe() would be buggy ...
> With that:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>
> Thanks,
> Ryan
>
> >
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Cc: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > ---
> > include/linux/pgtable.h | 6 ------
> > 1 file changed, 6 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 2a6a3cccfc36..aeabbf0db7c8 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1058,12 +1058,6 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
> > * same value. Otherwise, use the typical "set" helpers and flush the
> > * TLB.
> > */
> > -#define set_pte_safe(ptep, pte) \
> > -({ \
> > - WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
> > - set_pte(ptep, pte); \
> > -})
> > -
> > #define set_pmd_safe(pmdp, pmd) \
> > ({ \
> > WARN_ON_ONCE(pmd_present(*pmdp) && !pmd_same(*pmdp, pmd)); \
>
>
