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

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Date:	Mon, 1 Feb 2016 05:27:30 +1100
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Ross Zwisler <zwisler@...il.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Sun, Jan 31, 2016 at 10:18:46AM -0800, Dan Williams wrote:
> On Sun, Jan 31, 2016 at 10:07 AM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> > Yes.  This is just the default implementation of dax_map_pfn() which works
> > for most situations.  We can introduce more complex implementations of
> > dax_map_pfn() as necessary.  You make another excellent point for why
> > we should store PFNs in the radix tree instead of kaddrs :-)
> 
> How much complexity do we want to add in support of an fsync/msync
> mechanism that is not the recommended way to use DAX?

It actually makes the dax_io path much, much simpler.  And it's not
primarily about fixing fsync/msync.  It also makes the fault path cheaper
in the case where we're refaulting a page that's already been faulted
by another process (or was previously faulted by this process and now
needs to be faulted at a different address).

And it fixes the problem with filesystems that use multiple block_devices.
It also makes DAX much less reliant on buffer heads, which is good for
the problem that Jared raised where he doesn't have a block_device in
an embedded system.