linux-kernel - Re: [PATCH 5/5] block: enable dax for raw block devices

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Date:	Tue, 27 Oct 2015 09:19:30 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>, "hch@....de" <hch@....de>,
	"axboe@...com" <axboe@...com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"willy@...ux.intel.com" <willy@...ux.intel.com>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 5/5] block: enable dax for raw block devices

On Mon, Oct 26, 2015 at 05:56:30PM +0900, Dan Williams wrote:
> On Mon, Oct 26, 2015 at 3:23 PM, Dave Chinner <david@...morbit.com> wrote:
> > Also, DAX access isn't a property of mmap - it's a property
> > of the inode. We cannot do DAX access via mmap while mixing page
> > cache based access through file descriptor based interfaces. This
> > I why I'm adding an inode attribute (on disk) to enable per-file DAX
> > capabilities - either everything is via the DAX paths, or nothing
> > is.
> >
> 
> Per-inode control sounds very useful, I'll look at a similar mechanism
> for the raw block case.
> 
> However, still not quite convinced page-cache control is an inode-only
> property, especially when direct-i/o is not an inode-property.  That
> said, I agree the complexity of handling mixed mappings of the same
> file is prohibitive.

We didn't get that choice with direct IO - support via O_DIRECT was
kinda inherited from other OS's(*). We still have all sorts of
coherency problems between buffered/mmap/direct IO on the same file,
and I'd really, really like to avoid making that same mistake again
with DAX.

i.e. We have a choice with DAX right now that will allow us to avoid
coherency problems that we know existi and can't solve right now.
Making DAX and inode property rather than a application context
property avoids those coherence problems as all access will play by
the same rules....

(*)That said, some other OS's did O_DIRECT as an inode property (e.g.
solaris) where O_DIRECT was only done if no other cached operations
were required (e.g. mmap), and so the fd would transparently shift
between buffered and O_DIRECT depending on external accesses to the
inode. This was not liked because of it's unpredictable effect on
CPU usage and IO latency....

> Sounds good, get blkdev_issue_flush() functional first and then worry
> about building a more efficient solution on top.

*nod*

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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