Date:	Tue, 28 Apr 2015 11:05:48 +0200
From:	Beata Michalska <b.michalska@...sung.com>
To:	Greg KH <greg@...ah.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, jack@...e.cz, 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

Hi,

On 04/27/2015 05:37 PM, 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 :)
> 

The generic netlink interface is already being used by quota. As this is to
support file system events, including the threshold notifications, it just
seemed like a nice extension. I'm not really convinced that the concept here
goes well with the uevents and it's current usage. On the other hand, GFS2
already benefits form it. Still the generic netlink seems somehow ... lighter.
Anyway I'm open to any suggestions :)
  
> 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.)?
> 

I might be missing smth here, but any application might subscribe to
the multicast group so I'm not sure I understand your concerns here (?)

>>>> --- a/fs/Makefile +++ b/fs/Makefile @@ -126,3 +126,4 @@ obj-y 
>>>> += exofs/ # Multiple modules obj-$(CONFIG_CEPH_FS)		+= ceph/ 
>>>> obj-$(CONFIG_PSTORE)		+= pstore/ obj-$(CONFIG_EFIVAR_FS)		+= 
>>>> efivarfs/ +obj-y				+= events/
>>> 
>>> Always?
>>> 
>>>> diff --git a/fs/events/Makefile b/fs/events/Makefile new file 
>>>> mode 100644 index 0000000..58d1454 --- /dev/null +++ 
>>>> b/fs/events/Makefile @@ -0,0 +1,6 @@ +# +# Makefile for the 
>>>> Linux Generic File System Event Interface +# + +obj-y := 
>>>> fs_event.o
>>> 
>>> Always?  Even if the option is not selected?  Why is everyone 
>>> forced to always use this code?  Can't you disable it for the 
>>> "tiny" systems that don't need it?
>>> 
>> 
>> I was considering making it optional and I guess it's worth getting
>> back to this idea.
> 
> The "linux-tiny" people will appreciate that :)
> 

Consider it done for the next round.

>>>> +struct fs_trace_entry { +	atomic_t	 count;
>>> 
>>> Why not just use a 'struct kref' for your count, which will save
>>>  a bunch of open-coding of reference counting, and forcing us to
>>>  audit your code to verify you got all the corner cases correct?
>>>  :)
>>> 
>>>> +	atomic_t	 active; +	struct super_block *sb;
>> 
>> Not sure if using kref would change much here as the kref would not
>> really make it easier to verify those corner cases, unfortunately.
> 
> Why not, that's the goal of a kref.  Yes, you already did the hard 
> work, but now you require everyone else to also do the hard work of 
> trying to audit your code.  That's why we have common functions/data
>  structures in the kernel, to make long-term maintenance easier.
> 
> Please switch to make it so that we "know" you are doing this 
> correctly.
> 

Alright, if this is to make the review any easier I'll do that.
Still it's gonna be replacing fs_trace_entry_get/put with kref
and I doubt it will help verifying if the references are being
acquired / released properly.

>>> Are you properly reference counting this pointer?  I didn't see 
>>> where that was happening, so I must have missed it.
>>> 
>>> thanks,
>>> 
>> 
>> You haven't. And if I haven't missed anything, the sb is being used
>> only as long as the super is alive.
> 
> How do you know that? :)
> 
>> Most of the code operates on sb only if it was explicitly asked to,
>> through call from filesystem. There is also a callback notifying of
>> mount being dropped (which proceeds the call to kill_super) that
>> invalidates the object that depends on it. Still, it should be
>> explicitly stated that the sb is being used through bidding up the
>> s_count counter, though that would require taking the sb_lock.
>> AFAIK, one can get the reference to super block but for a
>> particular device. Maybe it would be worth having it more generic
>> (?).
> 
> Why not just grab a reference to the sb when you save the pointer, 
> and release it when you are done with it?  That should handle the 
> lifecycle properly.  It's always a very bad idea to have a pointer to
> a reference counted object without actually grabbing the reference,
> as you have no idea what is happening with it behind your back.
> 

Ok, will do.

> thanks,
> 
> greg k-h
> 

Thanks for your comments so far. Much appreciated.

BR
Beata
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