linux-kernel - Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

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Message-ID: <20160130052833.GY2948@linux.intel.com>
Date:	Sat, 30 Jan 2016 00:28:33 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-fsdevel@...r.kernel.org, linux-nvdimm@...1.01.org
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Fri, Jan 29, 2016 at 11:28:15AM -0700, Ross Zwisler wrote:
> I guess I need to go off and understand if we can have DAX mappings on such a
> device.  If we can, we may have a problem - we can get the block_device from
> get_block() in I/O path and the various fault paths, but we don't have access
> to get_block() when flushing via dax_writeback_mapping_range().  We avoid
> needing it the normal case by storing the sector results from get_block() in
> the radix tree.

I think we're doing it wrong by storing the sector in the radix tree; we'd
really need to store both the sector and the bdev which is too much data.

If we store the PFN of the underlying page instead, we don't have this
problem.  Instead, we have a different problem; of the device going
away under us.  I'm trying to find the code which tears down PTEs when
the device goes away, and I'm not seeing it.  What do we do about user
mappings of the device?