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Date:   Mon, 23 Jan 2017 06:37:18 +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>,
        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 Sun, Jan 22, 2017 at 06:39:28PM +0000, Matthew Wilcox wrote:
> > Two guests on the same physical machine (or a guest and a host) have access
> > to the same set of physical addresses.  This might be an NV-DIMM, or it might
> > just be DRAM (for the purposes of reducing guest overhead).  The network
> > filesystem has been enhanced with a call to allow the client to ask the server
> > "What is the physical address for this range of bytes in this file?"
> >
> > We don't want to use the guest pagecache here.  That's antithetical to the
> > second usage, and it's inefficient for the first usage.
> 
> And the answer is that you need a dax device for whatever memoery exposed
> in this way, as it needs to show up in the memory map for example.

Wow, DAX devices look painful and awful.  I certainly don't want to be exposing the memory fronted by my network filesystem to userspace to access.  That just seems like a world of pain and bad experiences.  Absolutely the filesystem (or perhaps better, the ACPI tables) need to mark that chunk of memory as reserved, but it's definitely not available for anyone to access without the filesystem being aware.

Even if we let the filesystem create a DAX device that doesn't show up in /dev (for example), Dan's patches don't give us a way to go from a file on the filesystem to a set of dax_ops.  And it does need to be a per-file operation, eg to support a file on an XFS volume which might be on a RT device or a normal device.  That was why I leaned towards an address_space operation, but I'd be happy to see an inode_operation instead. 

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