linux-ext4 - Re: [PATCH V9 03/11] fs/stat: Define DAX statx attribute

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Date:   Thu, 23 Apr 2020 10:24:22 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        Dave Chinner <dchinner@...hat.com>, Jan Kara <jack@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V9 03/11] fs/stat: Define DAX statx attribute

On Wed 22-04-20 11:51:21, Ira Weiny wrote:
> On Wed, Apr 22, 2020 at 09:29:51AM -0700, Darrick J. Wong wrote:
> > On Tue, Apr 21, 2020 at 12:17:45PM -0700, ira.weiny@...el.com wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> > > In order for users to determine if a file is currently operating in DAX
> > > state (effective DAX).  Define a statx attribute value and set that
> > > attribute if the effective DAX flag is set.
> > > 
> > > To go along with this we propose the following addition to the statx man
> > > page:
> > > 
> > > STATX_ATTR_DAX
> > > 
> > > 	The file is in the DAX (cpu direct access) state.  DAX state
> > > 	attempts to minimize software cache effects for both I/O and
> > > 	memory mappings of this file.  It requires a file system which
> > > 	has been configured to support DAX.
> > > 
> > > 	DAX generally assumes all accesses are via cpu load / store
> > > 	instructions which can minimize overhead for small accesses, but
> > > 	may adversely affect cpu utilization for large transfers.
> > > 
> > > 	File I/O is done directly to/from user-space buffers and memory
> > > 	mapped I/O may be performed with direct memory mappings that
> > > 	bypass kernel page cache.
> > > 
> > > 	While the DAX property tends to result in data being transferred
> > > 	synchronously, it does not give the same guarantees of O_SYNC
> > > 	where data and the necessary metadata are transferred together.
> > > 
> > > 	A DAX file may support being mapped with the MAP_SYNC flag,
> > > 	which enables a program to use CPU cache flush instructions to
> > > 	persist CPU store operations without an explicit fsync(2).  See
> > > 	mmap(2) for more information.
> > 
> > One thing I hadn't noticed before -- this is a change to userspace API,
> > so please cc this series to linux-api@...r.kernel.org when you send V10.
> 
> Right!  Glad you caught me on this because I was just preparing to send V10.
> 
> Is there someone I could directly mail who needs to look at this?  I guess I
> thought we had the important FS people involved for this type of API change.
> :-/

I believe we have all the important people here. But linux-api is a general
fallback list where people reviewing API changes linger. So when changing
user facing API, it is good to CC this list.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR