linux-kernel - Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to libnvdimm

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Date:   Mon, 23 Jan 2017 19:03:14 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Tony Luck <tony.luck@...el.com>, Jan Kara <jack@...e.cz>,
        Toshi Kani <toshi.kani@....com>,
        Mike Snitzer <snitzer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
        Jens Axboe <axboe@...com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to
        libnvdimm

On Mon, Jan 23, 2017 at 09:14:04AM -0800, Dan Williams wrote:
> The use case that we have now is distinguishing volatile vs persistent
> memory (brd vs pmem).

brd is a development tool, so until we have other reasons for this
abstraction (which I'm pretty sure will show up rather sooner than later)
I would not worry about it too much.

> I took a look at mtd layering approach and the main difference is that
> layers above the block layer do not appear to know anything about mtd
> specifics.

Or the block layer itself for that matter.  And that's exactly where
I want DAX to be in the future.

> For fs/dax.c we currently need some path to retrieve a dax
> anchor object through the block device.

We have a need to retreiver the anchor object.  We currently do it
though the block layer for historical reasons, but it doesn't have
to be that way.

> > In the longer run I like your dax_operations, but they need to be
> > separate from the block layer.
> 
> I'll move them from block_device_operations to dax data hanging off of
> the bdev_inode, or is there a better way to go from bdev-to-dax?

I don't think that's any better.  What we really want is a way
to find the underlying persistent memory / DAX / whatever we call
it node without going through a block device.  E.g. a library function
to give that object for a given path name, where the path name could
be either that of the /dev/pmemN or the /dev/daxN device.

If the file system for now still needs a block device as well it
will only accept the /dev/pmemN name, and open both the low-level
pmem device and the block device.  Once that file system doesn't
need block code (and I think we could do that easily for XFS,
nevermind any new FS) it won't have to deal with the block
device at all.

pmem.c then becomes a consumer of the dax_ops just like the file system.