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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