linux-kernel - Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ

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Message-Id: <22E0F870-C1FB-431E-BF6C-B395A09A2B0D@dilger.ca>
Date:	Fri, 13 Nov 2015 17:43:28 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	X86 ML <x86@...nel.org>, XFS Developers <xfs@....sgi.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler
> <ross.zwisler@...ux.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@...ux.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.

If the upper level filesystem gets an error when submitting a flush
request, then it assumes the underlying hardware is broken and cannot
be as aggressive in IO submission, but instead has to wait for in-flight
IO to complete.  Since FUA/FLUSH is basically a no-op for pmem devices,
it doesn't make sense _not_ to support this functionality.

Cheers, Andreas






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