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

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Date:	Tue, 2 Feb 2016 09:10:24 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	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>,
	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 Tue, Feb 2, 2016 at 8:46 AM, Jan Kara <jack@...e.cz> wrote:
> On Tue 02-02-16 08:33:56, Dan Williams wrote:
>> On Tue, Feb 2, 2016 at 3:17 AM, Jan Kara <jack@...e.cz> wrote:
>> [..]
>> > I see, thanks for explanation. So I'm OK with changing what is stored in
>> > the radix tree to accommodate this use case but my reservation that we IHMO
>> > have other more pressing things to fix remains...
>>
>> We don't need pfns in the radix to support XFS RT configurations.
>> Just call get_blocks() again and use the sector, or am I missing
>> something?
>
> You are correct. But if you decide to pay the cost of additional
> get_block() call, you only need the dirty tag in the radix tree and nothing
> else. So my understanding was that the whole point of games with radix tree
> is avoiding this extra get_block() calls for fsync().
>

DAX-fsync() is already a potentially expensive operation to cover data
durability guarantees for DAX-unaware applications.  A DAX-aware
application is going to skip fsync, and the get_blocks() cost, to do
cache management itself.

Willy pointed out some other potential benefits, assuming a suitable
replacement for the protections afforded by the block-device
percpu_ref counter can be found.  However, optimizing for the
DAX-unaware-application case seems the wrong motivation to me.