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Date: Mon, 16 Nov 2015 16:57:24 -0700
From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>, Jan Kara <jack@suse.cz>,
        Andreas Dilger <adilger@dilger.ca>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        Alexander Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>, Ingo Molnar <mingo@redhat.com>,
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        Matthew Wilcox <willy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling
Message-ID: <20151116235724.GA10256@linux.intel.com>
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	Matthew Wilcox <matthew.r.wilcox@intel.com>,
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On Mon, Nov 16, 2015 at 12:34:55PM -0800, Dan Williams wrote:
> On Mon, Nov 16, 2015 at 11:48 AM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > On Mon, Nov 16, 2015 at 09:28:59AM -0800, Dan Williams wrote:
> >> On Mon, Nov 16, 2015 at 6:05 AM, Jan Kara <jack@suse.cz> wrote:
> >> > On Mon 16-11-15 14:37:14, Jan Kara wrote:
> [..]
> > Is there any reason why this wouldn't work or wouldn't be a good idea?
> 
> We don't have numbers to support the claim that pcommit is so
> expensive as to need be deferred, especially if the upper layers are
> already taking the hit on doing the flushes.
> 
> REQ_FLUSH, means flush your volatile write cache.  Currently all I/O
> through the driver never hits a volatile cache so there's no need to
> tell the block layer that we have a volatile write cache, especially
> when you have the core mm taking responsibility for doing cache
> maintenance for dax-mmap ranges.
> 
> We also don't have numbers on if/when wbinvd is a more performant solution.
> 
> tl;dr Now that we have a baseline implementation can we please use
> data to make future arch decisions?

Sure, fair enough.