From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling
Date: Fri, 13 Nov 2015 16:20:39 -0800
Message-ID: <CAPcyv4j4arHE+iAALn1WPDzSb_QSCDy8udtXU1FV=kYSZDfv8A@mail.gmail.com>
References: <1447459610-14259-1-git-send-email-ross.zwisler@linux.intel.com>
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Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"J. Bruce Fields" <bfields@fieldses.org>, "Theodore Ts'o" <tytso@mit.edu>,
	Alexander Viro <viro@zeniv.linux.org.uk>, Andreas Dilger <adilger.kernel@dilger.ca>,
	Dave Chinner <david@fromorbit.com>, Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.com>,
	Jeff Layton <jlayton@poochiereds.net>, Matthew Wilcox <willy@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>, linux-ext4@vger.kernel.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, X86 ML <x86@kernel.org>, xfs@oss.sgi.com,
	Andrew Morton <akpm@linux-foundation.org>, Matthew Wilcox <matthew.r.wilcox@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
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On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios.  These
> are sent down via blkdev_issue_flush() in response to a fsync() or msync()
> and are used by filesystems to order their metadata, among other things.
>
> When we get an msync() or fsync() it is the responsibility of the DAX code
> to flush all dirty pages to media.  The PMEM driver then just has issue a
> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all
> the flushed data has been durably stored on the media.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Hmm, I'm not seeing why we need this patch.  If the actual flushing of
the cache is done by the core why does the driver need support
REQ_FLUSH?  Especially since it's just a couple instructions.  REQ_FUA
only makes sense if individual writes can bypass the "drive" cache,
but no I/O submitted to the driver proper is ever cached we always
flush it through to media.

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