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 [thread overview]
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
next prev parent reply other threads:[~2018-10-02 13:32 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 [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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