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

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Message-ID: <20160201145147.GD13740@quack.suse.cz>
Date:	Mon, 1 Feb 2016 15:51:47 +0100
From:	Jan Kara <jack@...e.cz>
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,
	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 Sat 30-01-16 00:28:33, Matthew Wilcox 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?

So I don't have a strong opinion whether storing PFN or sector is better.
Maybe PFN is somewhat more generic but OTOH turning DAX off for special
cases like inodes on XFS RT devices would be IMHO fine.

I'm somewhat concerned that there are several things in flight (page fault
rework, invalidation on device removal, issues with DAX access to block
devices Ross found) and this is IMHO the smallest trouble we have and changing
this seems relatively invasive. So could we settle the fault code and
similar stuff first and look into this somewhat later? Because frankly I
have some trouble following how all the pieces are going to fit together
and I'm afraid we'll introduce some non-trivial bugs when several
fundamental things are in flux in parallel.

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