On Fri, Nov 12, 2010 at 12:41:15PM -0800, Greg Thelen wrote: > >> mem_cgroup_page_stat() has changed so it never returns > >> error so convert the return value to the traditional page > >> count type (unsigned long). > > > > This changelog feels a bit beside the point. > > > > What's really interesting is that we now don't consider negative sums > > to be invalid anymore, but just assume zero! There is a real > > semantical change here. > > Prior to this patch series mem_cgroup_page_stat() returned a negative > value (specifically -EINVAL) to indicate that the current task was in > the root_cgroup and thus the per-cgroup usage and limit counter were > invalid. Callers treated all negative values as an indication of > root-cgroup message. > > Unfortunately there was another way that mem_cgroup_page_stat() could > return a negative value even when current was not in the root cgroup. > Negative sums were a possibility due to summing of unsynchronized > per-cpu counters. These occasional negative sums would fool callers > into thinking that the current task was in the root cgroup. > > Would adding this description to the commit message address your > concerns? I'd just describe that summing per-cpu counters is racy, that we can end up with negative results, and the only sensible handling of that is to assume zero. > > That the return type can then be changed to unsigned long is a nice > > follow-up cleanup that happens to be folded into this patch. > > Good point. I can separate the change into two sub-patches: > 1. use zero for a min-value (as described above) > 2. change return value to unsigned Sounds good. You can just fold the previous patch (adjusting the callsites) into 2, which should take care of the ordering problem. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>