On Fri, 8 Feb 2013, Gerald Schaefer wrote: > On Fri, 25 Jan 2013 18:10:18 -0800 (PST) > Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > Complaints are rare, but lockdep still does not understand the way > > ksm_memory_callback(MEM_GOING_OFFLINE) takes ksm_thread_mutex, and > > holds it until the ksm_memory_callback(MEM_OFFLINE): that appears > > to be a problem because notifier callbacks are made under down_read > > of blocking_notifier_head->rwsem (so first the mutex is taken while > > holding the rwsem, then later the rwsem is taken while still holding > > the mutex); but is not in fact a problem because mem_hotplug_mutex > > is held throughout the dance. > > > > There was an attempt to fix this with mutex_lock_nested(); but if that > > happened to fool lockdep two years ago, apparently it does so no > > longer. > > > > I had hoped to eradicate this issue in extending KSM page migration > > not to need the ksm_thread_mutex. But then realized that although > > the page migration itself is safe, we do still need to lock out ksmd > > and other users of get_ksm_page() while offlining memory - at some > > point between MEM_GOING_OFFLINE and MEM_OFFLINE, the struct pages > > themselves may vanish, and get_ksm_page()'s accesses to them become a > > violation. > > > > So, give up on holding ksm_thread_mutex itself from MEM_GOING_OFFLINE > > to MEM_OFFLINE, and add a KSM_RUN_OFFLINE flag, and > > wait_while_offlining() checks, to achieve the same lockout without > > being caught by lockdep. This is less elegant for KSM, but it's more > > important to keep lockdep useful to other users - and I apologize for > > how long it took to fix. > > Thanks a lot for the patch! I verified that it fixes the lockdep warning > that we got on memory hotremove. > > > > > Reported-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Thank you for reporting and testing and reporting back: sorry again for taking so long to fix it. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>