Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
> On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> > Hi Kirill,
> > 
> > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
> > <kirill.shutemov@linux.intel.com> wrote:
> > > DAX is not so special: we need i_mmap_lock to protect mapping->i_mmap.
> > >
> > > __dax_pmd_fault() uses unmap_mapping_range() shoot out zero page from
> > > all mappings. We need to drop i_mmap_lock there to avoid lock deadlock.
> > >
> > > Re-aquiring the lock should be fine since we check i_size after the
> > > point.
> > >
> > > Not-yet-signed-off-by: Matthew Wilcox <willy@linux.intel.com>
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > ---
> > >  fs/dax.c    | 35 +++++++++++++++++++----------------
> > >  mm/memory.c | 11 ++---------
> > >  2 files changed, 21 insertions(+), 25 deletions(-)
> > >
> > > diff --git a/fs/dax.c b/fs/dax.c
> > > index 9ef9b80cc132..ed54efedade6 100644
> > > --- a/fs/dax.c
> > > +++ b/fs/dax.c
> > > @@ -554,6 +554,25 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> > >         if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
> > >                 goto fallback;
> > >
> > > +       if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> > > +               int i;
> > > +               for (i = 0; i < PTRS_PER_PMD; i++)
> > > +                       clear_page(kaddr + i * PAGE_SIZE);
> > 
> > This patch, now upstream as commit 46c043ede471, moves the call to
> > clear_page() earlier in __dax_pmd_fault().  However, 'kaddr' is not
> > set at this point, so I'm not sure this path was ever tested.
> 
> Ughh. It's obviously broken.
> 
> I took fs/dax.c part of the patch from Matthew. And I'm not sure now we
> would need to move this "if (buffer_unwritten(&bh) || buffer_new(&bh)) {"
> block around. It should work fine where it was before. Right?
> Matthew?

Moving the "if (buffer_unwritten(&bh) || buffer_new(&bh)) {" block back seems
correct to me.  Matthew is out for a while, so we should probably take care of
this without him.

Kirill, do you want to whip up a quick patch?  I'm happy to do it if you're
busy.

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