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

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Date:   Sun, 22 Jan 2017 19:30:46 +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 06:19:24PM +0000, Matthew Wilcox wrote:
> No, I mean a network filesystem like 9p or cifs or nfs.  If the memcpy
> is supposed to be performed by the backing device

struct backing_dev has no relation to the DAX code.  Even more so what's
the point of doing a DAXish memcpy in that case?  If we buffer in
memory for network I/O we should just use the page cache.

> (Also, the network filesystem might have a command, like RDMA has/will have, to ensure that the write has reached persistence)

I know very well due to my work for a DAX-backed pNFS layout.  But that
is mostly transparent to the NFS frontend code and won't use DAX
on the client at all.  Just pagecache as a source for RDMA READ/WRITE.