Date:	Tue, 28 Apr 2015 19:39:00 +0200
From:	Greg KH <greg@...ah.com>
To:	Beata Michalska <b.michalska@...sung.com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	tytso@....edu, adilger.kernel@...ger.ca, hughd@...gle.com,
	lczerner@...hat.com, hch@...radead.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, kyungmin.park@...sung.com, kmpark@...radead.org
Subject: Re: [RFC v2 1/4] fs: Add generic file system event notifications

On Tue, Apr 28, 2015 at 04:46:46PM +0200, Beata Michalska wrote:
> On 04/28/2015 04:09 PM, Greg KH wrote:
> > On Tue, Apr 28, 2015 at 03:56:53PM +0200, Jan Kara wrote:
> >> On Mon 27-04-15 17:37:11, Greg KH wrote:
> >>> On Mon, Apr 27, 2015 at 05:08:27PM +0200, Beata Michalska wrote:
> >>>> On 04/27/2015 04:24 PM, Greg KH wrote:
> >>>>> On Mon, Apr 27, 2015 at 01:51:41PM +0200, Beata Michalska wrote:
> >>>>>> Introduce configurable generic interface for file
> >>>>>> system-wide event notifications, to provide file
> >>>>>> systems with a common way of reporting any potential
> >>>>>> issues as they emerge.
> >>>>>>
> >>>>>> The notifications are to be issued through generic
> >>>>>> netlink interface by newly introduced multicast group.
> >>>>>>
> >>>>>> Threshold notifications have been included, allowing
> >>>>>> triggering an event whenever the amount of free space drops
> >>>>>> below a certain level - or levels to be more precise as two
> >>>>>> of them are being supported: the lower and the upper range.
> >>>>>> The notifications work both ways: once the threshold level
> >>>>>> has been reached, an event shall be generated whenever
> >>>>>> the number of available blocks goes up again re-activating
> >>>>>> the threshold.
> >>>>>>
> >>>>>> The interface has been exposed through a vfs. Once mounted,
> >>>>>> it serves as an entry point for the set-up where one can
> >>>>>> register for particular file system events.
> >>>>>>
> >>>>>> Signed-off-by: Beata Michalska <b.michalska@...sung.com>
> >>>>>> ---
> >>>>>>  Documentation/filesystems/events.txt |  231 ++++++++++
> >>>>>>  fs/Makefile                          |    1 +
> >>>>>>  fs/events/Makefile                   |    6 +
> >>>>>>  fs/events/fs_event.c                 |  770 ++++++++++++++++++++++++++++++++++
> >>>>>>  fs/events/fs_event.h                 |   25 ++
> >>>>>>  fs/events/fs_event_netlink.c         |   99 +++++
> >>>>>>  fs/namespace.c                       |    1 +
> >>>>>>  include/linux/fs.h                   |    6 +-
> >>>>>>  include/linux/fs_event.h             |   58 +++
> >>>>>>  include/uapi/linux/fs_event.h        |   54 +++
> >>>>>>  include/uapi/linux/genetlink.h       |    1 +
> >>>>>>  net/netlink/genetlink.c              |    7 +-
> >>>>>>  12 files changed, 1257 insertions(+), 2 deletions(-)
> >>>>>>  create mode 100644 Documentation/filesystems/events.txt
> >>>>>>  create mode 100644 fs/events/Makefile
> >>>>>>  create mode 100644 fs/events/fs_event.c
> >>>>>>  create mode 100644 fs/events/fs_event.h
> >>>>>>  create mode 100644 fs/events/fs_event_netlink.c
> >>>>>>  create mode 100644 include/linux/fs_event.h
> >>>>>>  create mode 100644 include/uapi/linux/fs_event.h
> >>>>>
> >>>>> Any reason why you just don't do uevents for the block devices today,
> >>>>> and not create a new type of netlink message and userspace tool required
> >>>>> to read these?
> >>>>
> >>>> The idea here is to have support for filesystems with no backing device as well.
> >>>> Parsing the message with libnl is really simple and requires few lines of code
> >>>> (sample application has been presented in the initial version of this RFC)
> >>>
> >>> I'm not saying it's not "simple" to parse, just that now you are doing
> >>> something that requires a different tool.  If you have a block device,
> >>> you should be able to emit uevents for it, you don't need a backing
> >>> device, we handle virtual filesystems in /sys/block/ just fine :)
> >>>
> >>> People already have tools that listen to libudev for system monitoring
> >>> and management, why require them to hook up to yet-another-library?  And
> >>> what is going to provide the ability for multiple userspace tools to
> >>> listen to these netlink messages in case you have more than one program
> >>> that wants to watch for these things (i.e. multiple desktop filesystem
> >>> monitoring tools, system-health checkers, etc.)?
> >>   As much as I understand your concerns I'm not convinced uevent interface
> >> is a good fit. There are filesystems that don't have underlying block
> >> device - think of e.g. tmpfs or filesystems working directly on top of
> >> flash devices.  These still want to send notification to userspace (one of
> >> primary motivation for this interfaces was so that tmpfs can notify about
> >> something). And creating some fake nodes in /sys/block for tmpfs and
> >> similar filesystems seems like doing more harm than good to me...
> > 
> > If these are "fake" block devices, what's going to be present in the
> > block major/minor fields of the netlink message?  For some reason I
> > thought it was a required field, and because of that, I thought we had a
> > "real" filesystem somewhere to refer to, otherwise how would userspace
> > know what filesystem was creating these events?
> > 
> > What am I missing here?
> > 
> > confused,
> > 
> > greg k-h
> > 
> 
> For those 'fake' block devs, upon mount, get_anon_bdev will assign
> the major:minor numbers. Userspace might get those through stat.

How can userspace do the mapping backwards from this "anonymous"
major:minor number for these types of filesystems in such a way that
they can "know" how to report the block device that is causing the
event?

thanks,

greg k-h
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