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From: Jan Kara <jack@suse.cz>
To: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Jan Kara <jack@suse.cz>, Dan Williams <dan.j.williams@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	linux-nvdimm@lists.01.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Tue, 2 Oct 2018 16:29:59 +0200
Message-ID: <20181002142959.GD9127@quack2.suse.cz> (raw)
In-Reply-To: <20181002121039.GA3274@linux-x5ow.site>

[Added ext4, xfs, and linux-api folks to CC for the interface discussion]

On Tue 02-10-18 14:10:39, Johannes Thumshirn wrote:
> On Tue, Oct 02, 2018 at 12:05:31PM +0200, Jan Kara wrote:
> > Hello,
> > 
> > commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and device dax" has
> > removed VM_MIXEDMAP flag from DAX VMAs. Now our testing shows that in the
> > mean time certain customer of ours started poking into /proc/<pid>/smaps
> > and looks at VMA flags there and if VM_MIXEDMAP is missing among the VMA
> > flags, the application just fails to start complaining that DAX support is
> > missing in the kernel. The question now is how do we go about this?
> 
> OK naive question from me, how do we want an application to be able to
> check if it is running on a DAX mapping?

The question from me is: Should application really care? After all DAX is
just a caching decision. Sure it affects performance characteristics and
memory usage of the kernel but it is not a correctness issue (in particular
we took care for MAP_SYNC to return EOPNOTSUPP if the feature cannot be
supported for current mapping). And in the future the details of what we do
with DAX mapping can change - e.g. I could imagine we might decide to cache
writes in DRAM but do direct PMEM access on reads. And all this could be
auto-tuned based on media properties. And we don't want to tie our hands by
specifying too narrowly how the kernel is going to behave.

OTOH I understand that e.g. for a large database application the difference
between DAX and non-DAX mapping can be a difference between performs fine
and performs terribly / kills the machine so such application might want to
determine / force caching policy to save sysadmin from debugging why the
application is misbehaving.

> AFAIU DAX is always associated with a file descriptor of some kind (be
> it a real file with filesystem dax or the /dev/dax device file for
> device dax). So could a new fcntl() be of any help here? IS_DAX() only
> checks for the S_DAX flag in inode::i_flags, so this should be doable
> for both fsdax and devdax.

So fcntl() to query DAX usage is one option. Another option is the GETFLAGS
ioctl with which you can query the state of S_DAX flag (works only for XFS
currently). But that inode flag was meant more as a hint "use DAX if
available" AFAIK so that's probably not really suitable for querying
whether DAX is really in use or not. Since DAX is really about caching
policy, I was also thinking that we could use madvise / fadvise for this.
I.e., something like MADV_DIRECT_ACCESS which would return with success if
DAX is in use, with error if not. Later, kernel could use it as a hint to
really force DAX on a mapping and not try clever caching policies...
Thoughts?

								Honza

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

  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 [this message]
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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