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From: Johannes Thumshirn <jthumshirn@suse.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-nvdimm@lists.01.org, mhocko@suse.cz,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Tue, 2 Oct 2018 17:01:24 +0200
Message-ID: <20181002150123.GD4963@linux-x5ow.site> (raw)
In-Reply-To: <20181002144547.GA26735@infradead.org>

On Tue, Oct 02, 2018 at 07:45:47AM -0700, Christoph Hellwig wrote:
> How does an application "make use of DAX"?  What actual user visible
> semantics are associated with a file that has this flag set?

There may not be any user visible semantics of DAX, but there are
promises we gave to application developers praising DAX as _the_
method to map data on persistent memory and get around "the penalty of
the page cache" (however big this is).

As I said in another mail to this thread, applications have started to
poke in procfs to see whether they can use DAX or not.

Party A has promised party B something and they started checking for
it, then commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and
device dax" removed the way they checked if the kernel can keep up
with this promise.

So technically e1fb4a086495 is a user visible regression and in the
past we have reverted patches introducing these, even if the patch is
generally correct and poking in /proc/self/smaps is a bad idea.

I just wanted to give them a documented way to check for this
promise. Being neutral if this promise is right or wrong, good or bad,
or whatever. That's not my call, but I prefer not having angry users,
yelling at me because of broken applications.

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
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  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 [this message]
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
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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