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

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Message-ID: <20170509121444.GD21467@quack2.suse.cz>
Date:   Tue, 9 May 2017 14:14:44 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>, 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 Mon 08-05-17 11:25:27, Ross Zwisler wrote:
> 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>

Thanks. I'll add your reviewed-by tag and send patches to Andrew for
inclusion.
								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR