Re: [PATCH 4/4] dax: Fix data corruption when fault races with write

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Fri, May 05, 2017 at 09:25:00AM +0200, Jan Kara wrote:
> Currently DAX read fault can race with write(2) in the following way:
> 
> CPU1 - write(2)			CPU2 - read fault
> 				dax_iomap_pte_fault()
> 				  ->iomap_begin() - sees hole
> dax_iomap_rw()
>   iomap_apply()
>     ->iomap_begin - allocates blocks
>     dax_iomap_actor()
>       invalidate_inode_pages2_range()
>         - there's nothing to invalidate
> 				  grab_mapping_entry()
> 				  - we add zero page in the radix tree
> 				    and map it to page tables
> 
> The result is that hole page is mapped into page tables (and thus zeros
> are seen in mmap) while file has data written in that place.
> 
> Fix the problem by locking exception entry before mapping blocks for the
> fault. That way we are sure invalidate_inode_pages2_range() call for
> racing write will either block on entry lock waiting for the fault to
> finish (and unmap stale page tables after that) or read fault will see
> already allocated blocks by write(2).
> 
> Fixes: 9f141d6ef6258a3a37a045842d9ba7e68f368956
> CC: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Signed-off-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>

Yep, this looks correct to me.  Thanks!

Reviewed-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>