[PATCH] arm: mm: Fix definition of pmd_mknotpresent

[Will Deacon]

On Wed, Mar 02, 2016 at 06:11:52PM +0300, Kirill A. Shutemov wrote:
> On Wed, Mar 02, 2016 at 01:03:00AM +0000, Steve Capper wrote:
> > Currently pmd_mknotpresent will use a zero entry to respresent an
> > invalidated pmd.
> > 
> > Unfortunately this definition clashes with pmd_none, thus it is
> > possible for a race condition to occur if zap_pmd_range sees pmd_none
> > whilst __split_huge_pmd_locked is running too with pmdp_invalidate
> > just called.
> > 
> > This patch fixes the race condition by modifying pmd_mknotpresent to
> > create non-zero faulting entries (as is done in other architectures),
> > removing the ambiguity with pmd_none.
> > 
> > Fixes: 8d9625070073 ("ARM: mm: Transparent huge page support for LPAE systems.")
> > Reported-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> > Signed-off-by: Steve Capper <steve.capper at arm.com>
> > Acked-by: Will Deacon <will.deacon at arm.com>
> > 
> > ---
> > 
> > This patch addresses an issue discussed at:
> > http://lkml.iu.edu/hypermail/linux/kernel/1602.3/01846.html
> > 
> > ---
> >  arch/arm/include/asm/pgtable-3level.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/pgtable-3level.h b/arch/arm/include/asm/pgtable-3level.h
> > index dc46398..451b6e7 100644
> > --- a/arch/arm/include/asm/pgtable-3level.h
> > +++ b/arch/arm/include/asm/pgtable-3level.h
> > @@ -249,10 +249,10 @@ PMD_BIT_FUNC(mkyoung,   |= PMD_SECT_AF);
> >  #define pfn_pmd(pfn,prot)	(__pmd(((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot)))
> >  #define mk_pmd(page,prot)	pfn_pmd(page_to_pfn(page),prot)
> >  
> > -/* represent a notpresent pmd by zero, this is used by pmdp_invalidate */
> > +/* represent a notpresent pmd by faulting entry, this is used by pmdp_invalidate */
> >  static inline pmd_t pmd_mknotpresent(pmd_t pmd)
> >  {
> > -	return __pmd(0);
> > +	return __pmd(pmd_val(pmd) & ~PMD_TYPE_MASK);
> 
> I'm confused. Wouldn't it still be present?
> 
> arch/arm/include/asm/pgtable.h:#define pmd_present(pmd) (pmd_val(pmd))
> 
> Shouldn't pmd_present() be re-defined as (pmd_val(pmd) & PMD_TYPE_MASK) or
> something?

Hmm, we have ths same issue on arm64 in that case. Do we also need to
handle PROT_NONE for pmds?

Will