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

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Message-ID: <20170122162910.GA5267@lst.de>
Date:   Sun, 22 Jan 2017 17:29:10 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Christoph Hellwig <hch@....de>,
        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

On Sun, Jan 22, 2017 at 03:43:09PM +0000, Matthew Wilcox wrote:
> 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.

Again, do you mean block device?  For a filesystem that does not do any
pagecache writeback we already don't need a backing device, so I don't
really see an issue there to start with.

> 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.

I see no need to a new network filesystem to have a fake block device.
We do need a fake block device for an unchanged or partial DAX aware
file system.  And those are the only ones we have at the moment, although
XFS could be converted to do direct calls bypassing the block layer
fairly trivially if needed.  For ext2 and ext4 that would be much harder
due to the buffer cache dependency.