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

Kirill A. Shutemov
2016-10-11T21:47:26Z

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@linux.intel.com>
> > ---
> > 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@suse.cz>

Thanks!

--
Kirill A. Shutemov