On Thu, 2 Jun 2011, Andrea Arcangeli wrote: > On Thu, Jun 02, 2011 at 10:29:39AM -0700, Hugh Dickins wrote: > > AndreaA, I didn't study the patch you posted half an hour ago, > > since by that time I'd worked it out and was preparing patch below. > > I think your patch would be for a different bug, hopefully one we > > don't have, it looks more complicated than we should need for this. > > I didn't expect two different bugs leading to double free. There wasn't a double free there, just failure to cope with race emptying the list, so accessing head when expecting a full entry. > > If you've time please review my other patch too because mmput runs > with no mmap_sem hold and I think the ksm scan code runs under the > assumption that __ksm_exit is waiting in down_write() when > ksm_mmlist_lock is released (before freeing the mm_slot), and that > assumption is wrong. ksm_test_exit may very well be true despite > __ksm_exit didn't run yet, and ksm scan will proceed freeing after > changing the mm_slot and ksm_exit will be free to run and free again > immediately after the ksm scan releases the ksm_mmlist_lock and before > it clears the MMF_VM_MERGEABLE (because the mm_slot has been changed > before releasing the ksm_mmlist_lock). > > The rmap_list being null will kind of hide it, the fact there's so > little time between the unlock of the ksm_mmlist_lock and the clearing > of MMF_VM_MERGEABLE (that will stop ksm_exit from calling __ksm_exit > at all) will also hide it. At least in > unmerge_and_remove_all_rmap_items remove_trailing_rmap_items will nuke > the rmap_list just before this race runs so making it more likely > possible. You'll see from the "beware" comment in scan_get_next_rmap_item() that this case is expected, that it sometimes reaches freeing the slots before the exiting task reaches __ksm_exit(). That race should already be handled. I believe your patch is unnecessary, because get_mm_slot() is a hashlist lookup, and will return NULL once either end has done the hlist_del(&mm_slot->link). Hugh -- 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 internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>