From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH 1/2] fix bad behavior in use_hierarchy file Date: Tue, 26 Jun 2012 17:54:22 +0200 Message-ID: <20120626155422.GD27816@cmpxchg.org> References: <1340725634-9017-1-git-send-email-glommer@parallels.com> <1340725634-9017-2-git-send-email-glommer@parallels.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <1340725634-9017-2-git-send-email-glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Glauber Costa Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org, Michal Hocko , Andrew Morton , Dhaval Giani On Tue, Jun 26, 2012 at 07:47:13PM +0400, Glauber Costa wrote: > I have an application that does the following: > > * copy the state of all controllers attached to a hierarchy > * replicate it as a child of the current level. > > I would expect writes to the files to mostly succeed, since they > are inheriting sane values from parents. > > But that is not the case for use_hierarchy. If it is set to 0, we > succeed ok. If we're set to 1, the value of the file is automatically > set to 1 in the children, but if userspace tries to write the > very same 1, it will fail. That same situation happens if we > set use_hierarchy, create a child, and then try to write 1 again. > > Now, there is no reason whatsoever for failing to write a value > that is already there. It doesn't even match the comments, that > states: > > /* If parent's use_hierarchy is set, we can't make any modifications > * in the child subtrees... > > since we are not changing anything. > > The following patch tests the new value against the one we're storing, > and automatically return 0 if we're not proposing a change. > > Signed-off-by: Glauber Costa > CC: Dhaval Giani > CC: Michal Hocko > CC: Kamezawa Hiroyuki > CC: Johannes Weiner Acked-by: Johannes Weiner