Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Sep 18, 2019 at 05:00:27PM +0300, Kirill A. Shutemov wrote:
> On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote:
> > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
> >  	 */
> >  	if (unlikely(!src)) {
> >  		void *kaddr = kmap_atomic(dst);
> > -		void __user *uaddr = (void __user *)(va & PAGE_MASK);
> > +		void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> > +		pte_t entry;
> >  
> >  		/*
> >  		 * 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. On architectures with software "accessed" bits,
> > +		 * we would take a double page fault here, so mark it
> > +		 * accessed here.
> >  		 */
> > +		if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
> > +			spin_lock(vmf->ptl);
> > +			if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> > +				entry = pte_mkyoung(vmf->orig_pte);
> > +				if (ptep_set_access_flags(vma, addr,
> > +							  vmf->pte, entry, 0))
> > +					update_mmu_cache(vma, addr, vmf->pte);
> > +			}
> 
> I don't follow.
> 
> So if pte has changed under you, you don't set the accessed bit, but never
> the less copy from the user.
> 
> What makes you think it will not trigger the same problem?
> 
> I think we need to make cow_user_page() fail in this case and caller --
> wp_page_copy() -- return zero. If the fault was solved by other thread, we
> are fine. If not userspace would re-fault on the same address and we will
> handle the fault from the second attempt.

It would be nice to clarify the semantics of this function and do as
you suggest but the current comment is slightly confusing:

	/*
	 * If the source page was a PFN mapping, we don't have
	 * a "struct page" for it. We do a best-effort copy by
	 * just copying from the original user address. If that
	 * fails, we just zero-fill it. Live with it.
	 */

Would any user-space rely on getting a zero-filled page here instead of
a recursive fault?

-- 
Catalin