linux-ext4 - Re: [PATCH 4/4] dax: Fix data corruption when fault races with write

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Message-ID: <20170508172527.GA18408@linux.intel.com>
Date:   Mon, 8 May 2017 11:25:27 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nvdimm@...ts.01.org, stable@...r.kernel.org
Subject: Re: [PATCH 4/4] dax: Fix data corruption when fault races with write

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@...r.kernel.org
> Signed-off-by: Jan Kara <jack@...e.cz>

Yep, this looks correct to me.  Thanks!

Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>