Re: [PATCH kernfs/for-4.7-fixes] kernfs: don't depend on d_find_any_alias() when generating notifications

[Greg Kroah-Hartman]

On Tue, Jun 21, 2016 at 11:24:47AM -0400, Tejun Heo wrote:
> On Fri, Jun 17, 2016 at 05:51:17PM -0400, Tejun Heo wrote:
> > kernfs_notify_workfn() sends out file modified events for the
> > scheduled kernfs_nodes.  Because the modifications aren't from
> > userland, it doesn't have the matching file struct at hand and can't
> > use fsnotify_modify().  Instead, it looked up the inode and then used
> > d_find_any_alias() to find the dentry and used fsnotify_parent() and
> > fsnotify() directly to generate notifications.
> > 
> > The assumption was that the relevant dentries would have been pinned
> > if there are listeners, which isn't true as inotify doesn't pin
> > dentries at all and watching the parent doesn't pin the child dentries
> > even for dnotify.  This led to, for example, inotify watchers not
> > getting notifications if the system is under memory pressure and the
> > matching dentries got reclaimed.  It can also be triggered through
> > /proc/sys/vm/drop_caches or a remount attempt which involves shrinking
> > dcache.
> > 
> > fsnotify_parent() only uses the dentry to access the parent inode,
> > which kernfs can do easily.  Update kernfs_notify_workfn() so that it
> > uses fsnotify() directly for both the parent and target inodes without
> > going through d_find_any_alias().  While at it, supply the target file
> > name to fsnotify() from kernfs_node->name.
> > 
> > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> > Reported-by: Evgeny Vereshchagin <evvers@xxxxx>
> > Fixes: d911d9874801 ("kernfs: make kernfs_notify() trigger inotify events too")
> > Cc: John McCutchan <john@xxxxxxxxxxxxxxxxx>
> > Cc: Robert Love <rlove@xxxxxxxxx>
> > Cc: Eric Paris <eparis@xxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # v3.16+
> > ---
> > Hello,
> > 
> > I'm not sure this is the best way to deal with this but it at least
> > works fine.  If there's a better, please let me know.  If this
> > approach is okay, in the future, maybe we want to implement a helper
> > on fsnotify side to handle notification generation from back-end side?
> 
> Greg, can you please pick this one up?

Will do, thanks.

greg k-h