From: Jan Kara <jack@suse.cz>
To: Johannes Thumshirn <jthumshirn@suse.de>
Cc: linux-nvdimm@lists.01.org, linux-api@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org, Jan Kara <jack@suse.cz>,
linux-ext4@vger.kernel.org
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Tue, 2 Oct 2018 16:29:59 +0200 [thread overview]
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
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next prev parent reply other threads:[~2018-10-02 14:30 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-02 10:05 Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps 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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