On Thu, Mar 25, 2010 at 06:09:34PM +0900, KOSAKI Motohiro wrote: > > On Thu, 25 Mar 2010 08:32:35 +0000 > > Mel Gorman <mel@xxxxxxxxx> 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: > > > > 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. > > > > > Kosaki-san, > > > > IIUC, the race in memory-hotunplug was fixed by this patch [2/11]. > > > > But, this behavior of unmap_and_move() requires access to _freed_ > > objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU, > > it't not good habit in general. > > > > After direct compaction, page-migration will be one of "core" code of > > memory management. Then, I agree to patch [1/11] as our direction for > > keeping sanity and showing direction to more updates. Maybe adding > > refcnt and removing RCU in futuer is good. > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount > is meaningless now. Christoph is opposed to removing it because of cache-hotness issues more so than use-after-free concerns. The refcount is needed with or without SLAB_DESTROY_BY_RCU. > I agree you if we will remove SLAB_DESTROY_BY_RCU > in the future. > > refcount is easy understanding than rcu trick. > > > > IMHO, pushing this patch [2/11] as "BUGFIX" independent of this set and > > adding anon_vma->refcnt [1/11] and [3/11] in 1st Direct-compaction patch > > series to show the direction will makse sense. > > (I think merging 1/11 and 3/11 will be okay...) > > agreed. > > > > > Thanks, > > -Kame > > > > > > > -- 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>