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

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Date:   Sun, 22 Jan 2017 15:43:09 +0000
From:   Matthew Wilcox <mawilcox@...rosoft.com>
To:     Christoph Hellwig <hch@....de>
CC:     Dan Williams <dan.j.williams@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Brian Boylston <brian.boylston@....com>,
        "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

From: Christoph Hellwig [mailto:hch@....de]
> On Sat, Jan 21, 2017 at 04:28:52PM +0000, Matthew Wilcox wrote:
> > Of course, there may not be a backing device either!
> 
> s/backing device/block device/ ?  If so fully agreed.  I like the dax_ops
> scheme, but we should go all the way and detangle it from the block
> device.  I already brought up this issue with the fallback to direct I/O
> on I/O error series.

In the case of a network filesystem being used to communicate with a different VM on the same physical machine, there is no backing device, just a network protocol.
 
> And both of them are wrong.  The write_begin/write_end mistake
> notwithstanding address_space ops are operations the VM can call without
> knowing things like fs locking contexts.  The above on the other hand
> are device operations provided by the low-level driver, similar to
> block_device operations.  So what we need is to have a way to mount
> a dax device as a file system, similar to how we support that for block
> or MTD devices and can then call methods on it.  For now this will
> be a bit complicated because all current DAX-aware file systems also
> still need block device for the metadata path, so we can't just say
> you mount either a DAX or block device.  But I think we should aim
> for mounting a DAX device as the primary use case, and then deal
> with block device emulation as a generic DAX layer thing, similarly
> how we implement (bad in the rw case) block devices on top of MTD.

I'm not terribly enthusiastic about creating a fake block device to sit on top of a network filesystem, but I suppose we could go that way if we had to.