[PATCH 5/6] arm64: Ignore hardware dirty bit updates in ptep_set_wrprotect()

[Catalin Marinas]

On Thu, Aug 17, 2017 at 02:37:07PM +0100, Will Deacon wrote:
> On Tue, Jul 25, 2017 at 02:53:07PM +0100, Catalin Marinas wrote:
> > ptep_set_wrprotect() is only called on CoW mappings which are private
> > (!VM_SHARED) with the pte either read-only (!PTE_WRITE && PTE_RDONLY) or
> > writable and software-dirty (PTE_WRITE && !PTE_RDONLY && PTE_DIRTY).
> > There is no race with the hardware update of the dirty state: clearing
> > of PTE_RDONLY when PTE_WRITE (a.k.a. PTE_DBM) is set. This patch removes
> > the code setting the software PTE_DIRTY bit in ptep_set_wrprotect() as
> > superfluous. A VM_WARN_ONCE is introduced in case the above logic is
> > wrong or the core mm code changes its use of ptep_set_wrprotect().
> > 
> > Cc: Will Deacon <will.deacon at arm.com>
> > Acked-by: Steve Capper <steve.capper at arm.com>
> > Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index a14e2120811c..3fefcc0182c7 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -632,23 +632,28 @@ static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
> >  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> >  
> >  /*
> > - * ptep_set_wrprotect - mark read-only while trasferring potential hardware
> > - * dirty status (PTE_DBM && !PTE_RDONLY) to the software PTE_DIRTY bit.
> > + * ptep_set_wrprotect - mark read-only while preserving the hardware update of
> > + * the Access Flag.
> >   */
> >  #define __HAVE_ARCH_PTEP_SET_WRPROTECT
> >  static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long address, pte_t *ptep)
> >  {
> >  	pte_t old_pte, pte;
> >  
> > +	/*
> > +	 * ptep_set_wrprotect() is only called on CoW mappings which are
> > +	 * private (!VM_SHARED) with the pte either read-only (!PTE_WRITE &&
> > +	 * PTE_RDONLY) or writable and software-dirty (PTE_WRITE &&
> > +	 * !PTE_RDONLY && PTE_DIRTY); see is_cow_mapping() and
> > +	 * protection_map[]. There is no race with the hardware update of the
> > +	 * dirty state: clearing of PTE_RDONLY when PTE_WRITE (a.k.a. PTE_DBM)
> > +	 * is set.
> > +	 */
> > +	VM_WARN_ONCE(pte_write(*ptep) && !pte_dirty(*ptep),
> > +		     "%s: potential race with hardware DBM", __func__);
> 
> Just to confirm: but I take it the PTL serialises us against other threads
> trying to clean the pte?

Yes (see copy_pte_range()).

-- 
Catalin