Re: [PATCH 1/3] mm: New pfn_mkwrite same as page_mkwrite for VM_PFNMAP

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Mar 25, 2015 at 08:08:24AM -0700, Dave Hansen wrote:
> On 03/25/2015 06:38 AM, Boaz Harrosh wrote:
> >  /*
> >   * This routine handles present pages, when users try to write
> >   * to a shared page. It is done by copying the page to a new address
> > @@ -2025,8 +2042,17 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >  		 * accounting on raw pfn maps.
> >  		 */
> >  		if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> > -				     (VM_WRITE|VM_SHARED))
> > +				     (VM_WRITE|VM_SHARED)) {
> > +			pte_unmap_unlock(page_table, ptl);
> > +			ret = do_pfn_mkwrite(vma, address);
> > +			if (ret & VM_FAULT_ERROR)
> > +				return ret;
> > +			page_table = pte_offset_map_lock(mm, pmd, address,
> > +							 &ptl);
> > +			if (!pte_same(*page_table, orig_pte))
> > +				goto unlock;
> >  			goto reuse;
> > +		}
> >  		goto gotten;
> >  	}
> 
> This adds a lock release/reacquire in a place where the lock was
> previously just held.  Could you explain a bit why this is safe?

It's common practice in page fault codepath. See code around
->page_mkwrite for example.
> 
> Also, that pte_same() check looks a bit fragile.  It seems like it would
> fail if the hardware, for instance, set the accessed bit in here
> somewhere.  Is that what we want?

In this case we will cancel this fault handling and fault again. No
problems here.

-- 
 Kirill A. Shutemov

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