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From: Johannes Thumshirn <jthumshirn@suse.de>
To: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Jiang <dave.jiang@intel.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux MM <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Thu, 4 Oct 2018 12:04:06 +0200
Message-ID: <20181004100406.GE6682@linux-x5ow.site> (raw)
In-Reply-To: <20181003164407.GK24030@quack2.suse.cz>

On Wed, Oct 03, 2018 at 06:44:07PM +0200, Jan Kara wrote:
> On Wed 03-10-18 08:13:37, Dan Williams wrote:
> > On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <jack@suse.cz> wrote:
> > > WRT per-inode DAX property, AFAIU that inode flag is just going to be
> > > advisory thing - i.e., use DAX if possible. If you mount a filesystem with
> > > these inode flags set in a configuration which does not allow DAX to be
> > > used, you will still be able to access such inodes but the access will use
> > > page cache instead. And querying these flags should better show real
> > > on-disk status and not just whether DAX is used as that would result in an
> > > even bigger mess. So this feature seems to be somewhat orthogonal to the
> > > API I'm looking for.
> > 
> > True, I imagine once we have that flag we will be able to distinguish
> > the "saved" property and the "effective / live" property of DAX...
> > Also it's really not DAX that applications care about as much as "is
> > there page-cache indirection / overhead for this mapping?". That seems
> > to be a narrower guarantee that we can make than what "DAX" might
> > imply.
> 
> Right. So what do people think about my suggestion earlier in the thread to
> use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success
> when DAX is in use, failure otherwise. Later we could extend it to be also
> used as a hint for caching policy for the inode...

Hmm apart from Dan's objection that it can't really be used for a
query, isn't madvise(2) for mmap(2)?

But AFAIU (from looking at the xfs code, so please correct me if I',
wrong), DAX can be used for the traditional read(2)/write(2) interface
as well.

There is at least:

xfs_file_read_iter()
`-> if (IS_DAX(inode))
    `-> xfs_file_dax_read()
        `->dax_iomap_rw()

So IMHO something on an inode granularity would make more sens to me.

Byte,
	Johannes
-- 
Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 N�rnberg
GF: Felix Imend�rffer, Jane Smithard, Graham Norton
HRB 21284 (AG N�rnberg)
Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850

  parent reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-02 10:05 Jan Kara
2018-10-02 10:50 ` Michal Hocko
2018-10-02 13:32   ` Jan Kara
2018-10-02 12:10 ` Johannes Thumshirn
2018-10-02 14:20   ` Johannes Thumshirn
2018-10-02 14:45     ` Christoph Hellwig
2018-10-02 15:01       ` Johannes Thumshirn
2018-10-02 15:06         ` Christoph Hellwig
2018-10-04 10:09           ` Johannes Thumshirn
2018-10-05  6:25             ` Christoph Hellwig
2018-10-05  6:35               ` Johannes Thumshirn
2018-10-06  1:17                 ` Dan Williams
2018-10-14 15:47                   ` Dan Williams
2018-10-17 20:01                     ` Dan Williams
2018-10-18 17:43                       ` Jan Kara
2018-10-18 19:10                         ` Dan Williams
2018-10-19  3:01                           ` Dave Chinner
2018-10-02 14:29   ` Jan Kara
2018-10-02 14:37     ` Christoph Hellwig
2018-10-02 14:44       ` Johannes Thumshirn
2018-10-02 14:52         ` Christoph Hellwig
2018-10-02 15:31           ` Jan Kara
2018-10-02 20:18             ` Dan Williams
2018-10-03 12:50               ` Jan Kara
2018-10-03 14:38                 ` Dan Williams
2018-10-03 15:06                   ` Jan Kara
2018-10-03 15:13                     ` Dan Williams
2018-10-03 16:44                       ` Jan Kara
2018-10-03 21:13                         ` Dan Williams
2018-10-04 10:04                         ` Johannes Thumshirn [this message]
2018-10-02 15:07       ` Jan Kara
2018-10-17 20:23     ` Jeff Moyer
2018-10-18  0:25       ` Dave Chinner
2018-10-18 14:55         ` Jan Kara
2018-10-19  0:43           ` Dave Chinner
2018-10-30  6:30             ` Dan Williams
2018-10-30 22:49               ` Dave Chinner
2018-10-30 22:59                 ` Dan Williams
2018-10-31  5:59                 ` y-goto
2018-11-01 23:00                   ` Dave Chinner
2018-11-02  1:43                     ` y-goto
2018-10-18 21:05         ` Jeff Moyer
2018-10-09 19:43 ` Jeff Moyer
2018-10-16  8:25   ` Jan Kara
2018-10-16 12:35     ` Jeff Moyer

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