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From: Jan Kara <jack@suse.cz>
To: Michal Hocko <mhocko@kernel.org>
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, jthumshirn@suse.de
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Tue, 2 Oct 2018 15:32:04 +0200
Message-ID: <20181002133204.GC9127@quack2.suse.cz> (raw)
In-Reply-To: <20181002105058.GE18342@dhcp22.suse.cz>

On Tue 02-10-18 12:50:58, Michal Hocko wrote:
> On Tue 02-10-18 12:05:31, 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?
> 
> Do they need to check for a general DAX support or do they need a per
> mapping granularity?

General DAX support per filesystem is OK for them, at least for now. So
they might be OK with just checking for 'dax' mount option in /proc/mounts.
But I agree this is cumbersome.

> > Strictly speaking, this is a userspace visible regression (as much as I
> > think that application poking into VMA flags at this level is just too
> > bold). Is there any precedens in handling similar issues with smaps which
> > really exposes a lot of information that is dependent on kernel
> > implementation details?
> 
> Yeah, exposing all the vma flags was just a terrible idea. We have had a
> similar issue recently [1] for other flag that is no longer set while
> the implementation of the feature is still in place. I guess we really
> want to document that those flags are for debugging only and no stable
> and long term API should rely on it.

Yeah, I have some doubts about usefulness of such documentation but I guess
it's better than nothing.

> Considering how new the thing really is (does anybody do anything
> production like out there?) I would tend to try a better interface
> rather than chasing after random vma flags. E.g. what prevents a
> completely unrelated usage of VM_MIXEDMAP?

Nothing checking that flag is in production AFAIK but DAX as such is in
active use for some limited usecases already. I'll reply regarding a better
interface for checking DAX, in an email to Johannes.

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

  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 [this message]
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
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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