linux-ext4 - Re: [RFC 1/4] fs: Add generic file system event notifications

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Date:	Thu, 16 Apr 2015 10:41:58 +0200
From:	Beata Michalska <b.michalska@...sung.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-kernel@...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 1/4] fs: Add generic file system event notifications

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.

BR
Beata


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