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    SubjectRe: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX
    Date
    Ross Zwisler wrote:
    > 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.

    I would be better if you'll prepare the patch. Thanks.

    --
    Kirill


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