On Thu, Mar 25, 2010 at 05:56:25PM +0900, KOSAKI Motohiro wrote: > > On Thu, Mar 25, 2010 at 11:49:23AM +0900, KOSAKI Motohiro wrote: > > > > On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote: > > > > > > > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount. > > > > > > > So, I think we don't need your [1/11] patch. > > > > > > > > > > > > > > Am I missing something? > > > > > > > > > > > > > > > > > > > The refcount is still needed. The anon_vma might be valid, but the > > > > > > refcount is what ensures that the anon_vma is not freed and reused. > > > > > > > > > > please please why do we need both mechanism. now cristoph is very busy and I am > > > > > de fact reviewer of page migration and mempolicy code. I really hope to understand > > > > > your patch. > > > > > > > > As in, why not drop the RCU protection of anon_vma altogeter? Mainly, because I > > > > think it would be reaching too far for this patchset and it should be done as > > > > a follow-up. Putting the ref-count everywhere will change the cache-behaviour > > > > of anon_vma more than I'd like to slip into a patchset like this. Secondly, > > > > Christoph mentions that SLAB_DESTROY_BY_RCU is used to keep anon_vma cache-hot. > > > > For these reasons, removing RCU from these paths and adding the refcount > > > > in others is a patch that should stand on its own. > > > > > > Hmmm... > > > I haven't understand your mention because I guess I was wrong. > > > > > > probably my last question was unclear. I mean, > > > > > > 1) If we still need SLAB_DESTROY_BY_RCU, why do we need to add refcount? > > > Which difference is exist between normal page migration and compaction? > > > > The processes typically calling migration today own the page they are moving > > and is not going to exit unexpectedly during migration. > > > > > 2) If we added refcount, which race will solve? > > > > > > > The process exiting and the last anon_vma being dropped while compaction > > is running. This can be reliably triggered with compaction. > > > > > IOW, Is this patch fix old issue or compaction specific issue? > > > > Strictly speaking, it's an old issue but in practice it's impossible to > > trigger because the process migrating always owns the page. Compaction > > moves pages belonging to arbitrary processes. > > Do you mean current memroy hotplug code is broken??? I hadn't considered the memory hotplug case but you're right, it's possible it's at risk. While compaction can trigger this problem reliably, it's not exactly easy to trigger. I was triggering it under very heavy memory load with a large number of very short lived processes (specifically, an excessive compile-based load). It's possible that memory hotplug has not been tested under similar situations. > I think compaction need refcount, hotplug also need it. both they migrate another > task's page. > > but , I haven't seen hotplug failure. Am I missing something? or the compaction > have its specific race situation? > It's worth double-checking. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>