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

[Beata Michalska <b.michalska@samsung.com>]

On 04/17/2015 03:04 PM, Beata Michalska wrote:
> On 04/17/2015 01:31 PM, Jan Kara wrote:
>> 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@samsung.com>
>>   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@samsung.com>
>>> +
>>> +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?
> 
> No particular reason here - just for possible future extension if needed.
> I'm totally fine with having a single sysfs entry.
> 

On the other hand .... sysfs entries are mostly single-valued or are sets
of values of a single type, so not sure if we would fit in here -
with the current configuration for the interface.

>>
>>> +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).
>>
> Even if a given filesystem is being mounted in many places this will come
> down to single super_block - the interface will add trace for the first mount
> point. This is just to ease the usage: internally a particular trace is 
> associated with a super_block. 
> 
>> 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.
> 
> Alright then, so dropping the idea of announcing new trace (with assigned id)
> and switching to using the major:minor numbers. Sounds OK to me.
> 
>>
>>> + 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.
>>
> 
> This is not necessary but could ease the usage - i.e. through scripting: to specify
> multiple traces and register them in one go. 
> 
>> 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...
>>
> 
> Currently only the threshold events use arguments -  not sure what arguments
> could be used for the remaining notifications. But any suggestions are welcomed.
> 
>> ...
>>> +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
> 
> Getting rid of those would simplify the configuration part, indeed.
> So we would be left with 'generic' and threshold events.
> I guess I've overdone this part.
> 
> Thanks for Your comments so far.
> 
> BR
> Beata
> 
> 
> 
> 
> 

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