linux-kernel - Re: [PATCH] mm: take i_mmap_lock in unmap_mapping

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Date:	Wed, 16 Sep 2015 14:12:18 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dan Williams <dan.j.williams@...el.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
	ross.zwisler@...ux.intel.com
Subject: Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

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@...ux.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@...ux.intel.com>
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.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?

> I'm also not sure why the compiler is not complaining about an
> uninitialized variable?

No idea.

-- 
 Kirill A. Shutemov
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