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

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Date:	Thu, 16 Apr 2015 13:10:23 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Beata Michalska <b.michalska@...sung.com>
cc:	Eric Sandeen <sandeen@...hat.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	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 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@...r.kernel.org next time you post these.)

Hugh
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