Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared

[Catalin Marinas]

On Fri, Sep 06, 2019 at 07:57:42AM -0700, Matthew Wilcox wrote:
> On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> >  		 * This really shouldn't fail, because the page is there
> >  		 * in the page tables. But it might just be unreadable,
> >  		 * in which case we just give up and fill the result with
> > -		 * zeroes.
> > +		 * zeroes. If PTE_AF is cleared on arm64, it might
> > +		 * cause double page fault. So makes pte young here
> 
> How about:
> 		 * zeroes. On architectures with software "accessed" bits,
> 		 * we would take a double page fault here, so mark it
> 		 * accessed here.
> 
> >  		 */
> > +		if (!pte_young(vmf->orig_pte)) {
> 
> Let's guard this with:
> 
> 		if (arch_sw_access_bit && !pte_young(vmf->orig_pte)) {
> 
> #define arch_sw_access_bit	0
> by default and have arm64 override it (either to a variable or a constant
> ... your choice).  Also, please somebody decide on a better name than
> arch_sw_access_bit.

I'm not good at names either (is arch_faults_on_old_pte any better?) but
I'd make this a 0 args call: arch_sw_access_bit(). This way we can make
it a static inline function on arm64 with some static label check.

-- 
Catalin