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

[Tejun Heo <tj@kernel.org>]

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@kernel.org>
> Reported-by: Evgeny Vereshchagin <evvers@ya.ru>
> Fixes: d911d9874801 ("kernfs: make kernfs_notify() trigger inotify events too")
> Cc: John McCutchan <john@johnmccutchan.com>
> Cc: Robert Love <rlove@rlove.org>
> Cc: Eric Paris <eparis@parisplace.org>
> Cc: stable@vger.kernel.org # 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?

Thanks.

-- 
tejun