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

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Date:	Fri, 29 Jan 2016 22:01:13 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Fri, Jan 29, 2016 at 9:28 PM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> 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?
>

I deferred the dax tear down code until next cycle as Al rightly
pointed out some needed re-works:

https://lists.01.org/pipermail/linux-nvdimm/2016-January/003995.html