Re: [GIT PULL] Fsnotify cleanups

[Jan Kara <jack@suse.cz>]

On Sat 09-06-18 11:00:17, Amir Goldstein wrote:
> On Sat, Jun 9, 2018 at 9:57 AM, Amir Goldstein <amir73il@gmail.com> wrote:
> > The only code that needs to test the shadow mask before following into
> > connector is the optimization code in start of fsnotify() which was not
> > generalized anyway and where the mask optimization is more important
> > for mount watches.
> >
> 
> No. That's not true, there is also fsnotify_inode_watches_children() and
> __fsnotify_parent(). But I think the only bits in the mask that really matter
> for optimizing inode watches are FS_ACCESS and FS_EVENT_ON_CHILD
> and those could be encoded in lower bits of connector pointer.

Actually for children events, we have a dentry flag
DCACHE_FSNOTIFY_PARENT_WATCHED and that should catch 99% of situation.
FS_EVENT_ON_CHILD is used only to verify event really should be delivered
since the DCACHE_FSNOTIFY_PARENT_WATCHED flag is cleared lazily. Or where
do you think the performance of FS_EVENT_ON_CHILD checking matters?

Also I'm not 100% sure just having FS_ACCESS checked quickly is going to
buy us that much. We have quite a few event types and if someone is
watching for a different event type than the current event (just not
FS_ACCESS), we'll just have to pay the cost of srcu_read_lock and
indirection. So it just depends whether we consider such loads worthwhile
or not...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR