linux-kernel - Re: [PATCH] mm, dax: fix DAX deadlocks (COW fault)

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Date:	Mon, 16 Nov 2015 11:34:04 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Yigal Korman <yigal@...xistor.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Linux MM <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	david <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stable Tree <stable@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <dchinner@...hat.com>, Jan Kara <jack@...e.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: Re: [PATCH] mm, dax: fix DAX deadlocks (COW fault)

On Mon, Nov 16, 2015 at 10:15:56AM -0800, Dan Williams wrote:
> On Mon, Nov 16, 2015 at 4:09 AM, Yigal Korman <yigal@...xistor.com> wrote:
> > DAX handling of COW faults has wrong locking sequence:
> >         dax_fault does i_mmap_lock_read
> >         do_cow_fault does i_mmap_unlock_write
> >
> > Ross's commit[1] missed a fix[2] that Kirill added to Matthew's
> > commit[3].
> >
> > Original COW locking logic was introduced by Matthew here[4].
> >
> > This should be applied to v4.3 as well.
> >
> > [1] 0f90cc6609c7 mm, dax: fix DAX deadlocks
> > [2] 52a2b53ffde6 mm, dax: use i_mmap_unlock_write() in do_cow_fault()
> > [3] 843172978bb9 dax: fix race between simultaneous faults
> > [4] 2e4cdab0584f mm: allow page fault handlers to perform the COW
> >
> > Signed-off-by: Yigal Korman <yigal@...xistor.com>
> >
> > Cc: Stable Tree <stable@...r.kernel.org>
> > Cc: Boaz Harrosh <boaz@...xistor.com>
> > Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: Dave Chinner <dchinner@...hat.com>
> > Cc: Jan Kara <jack@...e.com>
> > Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > Cc: Matthew Wilcox <matthew.r.wilcox@...el.com>
> > ---
> >  mm/memory.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c716913..e5071af 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3015,9 +3015,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> >                 } else {
> >                         /*
> >                          * The fault handler has no page to lock, so it holds
> > -                        * i_mmap_lock for write to protect against truncate.
> > +                        * i_mmap_lock for read to protect against truncate.
> >                          */
> > -                       i_mmap_unlock_write(vma->vm_file->f_mapping);
> > +                       i_mmap_unlock_read(vma->vm_file->f_mapping);
> >                 }
> >                 goto uncharge_out;
> >         }
> > @@ -3031,9 +3031,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> >         } else {
> >                 /*
> >                  * The fault handler has no page to lock, so it holds
> > -                * i_mmap_lock for write to protect against truncate.
> > +                * i_mmap_lock for read to protect against truncate.
> >                  */
> > -               i_mmap_unlock_write(vma->vm_file->f_mapping);
> > +               i_mmap_unlock_read(vma->vm_file->f_mapping);
> >         }
> >         return ret;
> >  uncharge_out:
> 
> Looks good to me.  I'll include this with some other DAX fixes I have pending.

Looks good to me as well.  Thanks for catching this.
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