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

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

Hi,

On 04/16/2015 10:10 PM, Hugh Dickins wrote:
> On Thu, 16 Apr 2015, Beata Michalska wrote:
>> On 04/16/2015 05:46 AM, Eric Sandeen wrote:
>>> On 4/15/15 2:15 AM, 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.
>>>
>>> ...
>>>
>>>> + 4.3 Threshold notifications:
>>>> +
>>>> + #include <linux/fs_event.h>
>>>> + void fs_event_alloc_space(struct super_block *sb, u64 ncount);
>>>> + void fs_event_free_space(struct super_block *sb, u64 ncount);
>>>> +
>>>> + Each filesystme supporting the treshold notifiactions should call
>>>> + fs_event_alloc_space/fs_event_free_space repsectively whenever the
>>>> + ammount of availbale blocks changes.
>>>> + - sb:     the filesystem's super block
>>>> + - ncount: number of blocks being acquired/released
>>>
>>> so:
>>>
>>>> +void fs_event_alloc_space(struct super_block *sb, u64 ncount)
>>>> +{
>>>> +	struct fs_trace_entry *en;
>>>> +	s64 count;
>>>> +
>>>> +	spin_lock(&fs_trace_lock);
>>>
>>> Every allocation/free for every supported filesystem system-wide will be
>>> serialized on this global spinlock?  That sounds like a non-starter...
>>>
>>> -Eric
>>>
>> I guess there is a plenty room for improvements as this is an early version.
>> I do agree that this might be a performance bottleneck event though I've tried
>> to keep this to minimum - it's being taken only for hashtable look-up. But still...
>> I was considering placing the trace object within the super_block to skip
>> this look-up part but I'd like to gather more comments, especially on the concept
>> itself.
> 
> Sorry, I have no opinion on the netlink fs notifications concept
> itself, not my area of expertise at all.
> 
> No doubt you Cc'ed me for tmpfs: I am very glad you're now trying the
> generic filesystem route, and yes, I'd be happy to have the support
> in tmpfs, thank you - if it is generally agreed to be suitable for
> filesystems; but wouldn't want this as a special for tmpfs.
> 
> However, I must echo Eric's point: please take a look at 7e496299d4d2
> "tmpfs: make tmpfs scalable with percpu_counter for used blocks":
> Tim would be unhappy if you added overhead back into that path.
> 
> (And please Cc linux-fsdevel@vger.kernel.org next time you post these.)
> 
> Hugh
> 

Well, the concept of using netlink interface here is just a part of the overall
idea - so any comments are really welcomed here. The more of them the better solution
can be worked out, as I believe.

As for the possible overhead: this is the last thing I would want, so I'll
definitely do may best to not to introduce any. I will definitely rework this.

Thanks for Your comments,

BR
Beata