Re: [PATCHv3 12/41] thp: handle write-protection faults for file THP

[Kirill A. Shutemov]

On Tue, Oct 11, 2016 at 05:47:50PM +0200, Jan Kara wrote:
> On Thu 15-09-16 14:54:54, Kirill A. Shutemov wrote:
> > For filesystems that wants to be write-notified (has mkwrite), we will
> > encount write-protection faults for huge PMDs in shared mappings.
> > 
> > The easiest way to handle them is to clear the PMD and let it refault as
> > wriable.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  mm/memory.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 83be99d9d8a1..aad8d5c6311f 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3451,8 +3451,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
> >  		return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
> >  				fe->flags);
> >  
> > +	if (fe->vma->vm_flags & VM_SHARED) {
> > +		/* Clear PMD */
> > +		zap_page_range_single(fe->vma, fe->address,
> > +				HPAGE_PMD_SIZE, NULL);
> > +		VM_BUG_ON(!pmd_none(*fe->pmd));
> > +
> > +		/* Refault to establish writable PMD */
> > +		return 0;
> > +	}
> > +
> 
> Since we want to write-protect the page table entry on each page writeback
> and write-enable then on the next write, this is relatively expensive.
> Would it be that complicated to handle this fully in ->pmd_fault handler
> like we do for DAX?
> 
> Maybe it doesn't have to be done now but longer term I guess it might make
> sense.

Right. This approach is just simplier to implement. We can rework it if it
will show up on traces.

> Otherwise the patch looks good so feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Thanks!

-- 
 Kirill A. Shutemov