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. 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...) Thanks, -Kame -- 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>