Re: [RFC 1/4] fs: Add generic file system event notifications

[Jan Kara]

On Wed 15-04-15 09:15:44, 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 a dedicated, for file system
> events, multicast group. The file systems might as
> well use this group to send their own custom messages.
> 
> The events have been split into four base categories:
> information, warnings, errors and threshold notifications,
> with some very basic event types like running out of space
> or file system being remounted as read-only.
> 
> Threshold notifications have been included to allow
> 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@xxxxxxxxxxx>
  Thanks for the patches! Some comments are below.

> ---
>  Documentation/filesystems/events.txt |  254 +++++++++++
>  fs/Makefile                          |    1 +
>  fs/events/Makefile                   |    6 +
>  fs/events/fs_event.c                 |  775 ++++++++++++++++++++++++++++++++++
>  fs/events/fs_event.h                 |   27 ++
>  fs/events/fs_event_netlink.c         |   94 +++++
>  fs/namespace.c                       |    1 +
>  include/linux/fs.h                   |    6 +-
>  include/linux/fs_event.h             |   69 +++
>  include/uapi/linux/fs_event.h        |   62 +++
>  include/uapi/linux/genetlink.h       |    1 +
>  net/netlink/genetlink.c              |    7 +-
>  12 files changed, 1301 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
> 
> diff --git a/Documentation/filesystems/events.txt b/Documentation/filesystems/events.txt
> new file mode 100644
> index 0000000..c85dd88
> --- /dev/null
> +++ b/Documentation/filesystems/events.txt
> @@ -0,0 +1,254 @@
> +
> +	Generic file system event notification interface
> +
> +Document created 09 April 2015 by Beata Michalska <b.michalska@xxxxxxxxxxx>
> +
> +1. The reason behind:
> +=====================
> +
> +There are many corner cases when things might get messy with the filesystems.
> +And it is not always obvious what and when went wrong. Sometimes you might
> +get some subtle hints that there is something going on - but by the time
> +you realise it, it might be too late as you are already out-of-space
> +or the filesystem has been remounted as read-only (i.e.). The generic
> +interface for the filesystem events fills the gap by providing a rather
> +easy way of real-time notifications triggered whenever something intreseting
> +happens, allowing filesystems to report events in a common way, as they occur.
> +
> +2. How does it work:
> +====================
> +
> +The interface itself has been exposed as fstrace-type Virtual File System,
> +primarily to ease the process of setting up the configuration for the file
> +system notifications. So for starters it needs to get mounted (obviously):
> +
> +	mount -t fstrace none /sys/fs/events
> +
> +This will unveil the single fstrace filesystem entry - the 'config' file,
> +through which the notification are being set-up.
> +
> +Activating notifications for particular filesystem is as straightforward
> +as writing into the 'config' file. Note that by default all events despite
> +the actual filesystem type are being disregarded.
  Is there a reason to have a special filesystem for this? Do you expect
extending it by (many) more files? Why not just creating a file in sysfs or
something like that?

> +Synopsis of config:
> +------------------
> +
> +	MOUNT EVENT_TYPE [L1] [L2]
> +
> + MOUNT      : the filesystem's mount point
  I'm not quite decided but is mountpoint really the right thing to pass
via the interface? They aren't unique (filesystem can be mounted in
multiple places) and more importantly can change over time. So won't it be
better to pass major:minor over the interface? These are stable, unique to
the filesystem, and userspace can easily get them by calling stat(2) on the
desired path (or directly from /proc/self/mountinfo). That could be also
used as an fs identifier instead of assigned ID (and thus we won't need
those events about creation of new trace which look somewhat strange to
me).

OTOH using major:minor may have issues in container world where processes
could watch events from filesystems inaccessible to the container if they
guess the device number. So maybe we could use 'path' when creating new
trace but I'd still like to use the device number internally and for all
outgoing communication because of above mentioned problems with
mountpoints.

> + EVENT_TYPE : type of events to be enabled: info,warn,err,thr;
> +              at least one type needs to be specified;
> +              note the comma delimiter and lack of spaces between
> +	      those options
> + L1         : the threshold limit - lower range
> + L2         : the threshold limit - upper range
> + 	      case enabling threshold notifications the lower level is
> +	      mandatory, whereas the upper one remains optional;
> +	      note though, that as those refer to the number of available
> +	      blocks, the lower level needs to be higher than the upper one
> +
> +Sample request could look like the follwoing:
> +
> + echo /sample/mount/point warn,err,thr 710000 500000 > /sys/fs/events/config
> +
> +Multiple request might be specified provided they are separated with semicolon.
  Is this necessary? It somewhat complicates syntax and parsing in kernel
and I don't see a need for that. I'd prefer to keep the interface as simple
as possible.

Also I think that we should make it clear that each event type has
different set of arguments. For threshold events they'll be L1 & L2, for
other events there may be no arguments, for other events maybe something
else...

...
> +static const match_table_t fs_etypes = {
> +	{ FS_EVENT_INFO,    "info"  },
> +	{ FS_EVENT_WARN,    "warn"  },
> +	{ FS_EVENT_THRESH,  "thr"   },
> +	{ FS_EVENT_ERR,     "err"   },
> +	{ 0, NULL },
> +};
  Why are there these generic message types? Threshold messages make good
sense to me. But not so much the rest. If they don't have a clear meaning,
it will be a mess. So I also agree with a message like - "filesystem has
trouble, you should probably unmount and run fsck" - that's fine. But
generic "info" or "warning" doesn't really carry any meaning on its own and
thus seems pretty useless to me. To explain a bit more, AFAIU this
shouldn't be a generic logging interface where something like severity
makes sense but rather a relatively specific interface notifying about
events in filesystem userspace should know about so I expect relatively low
number of types of events, not tens or even hundreds...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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