on 2010-3-5 20:03, Paul Menage wrote: > On Wed, Mar 3, 2010 at 2:52 AM, Miao Xie <miaox@xxxxxxxxxxxxxx> wrote: >> if MAX_NUMNODES > BITS_PER_LONG, loading/storing task->mems_allowed or mems_allowed in >> task->mempolicy are not atomic operations, and the kernel page allocator gets an empty >> mems_allowed when updating task->mems_allowed or mems_allowed in task->mempolicy. So we >> use a rwlock to protect them to fix this probelm. > > Rather than adding locks, if the intention is just to avoid the > allocator seeing an empty nodemask couldn't we instead do the > equivalent of: > > current->mems_allowed |= new_mask; > current->mems_allowed = new_mask; > > i.e. effectively set all new bits in the nodemask first, and then > clear all old bits that are no longer in the new mask. The only > downside of this is that a page allocation that races with the update > could potentially allocate from any node in the union of the old and > new nodemasks - but that's the case anyway for an allocation that > races with an update, so I don't see that it's any worse. Before applying this patch, cpuset updates task->mems_allowed just like what you said. But the allocator is still likely to see an empty nodemask. This problem have been pointed out by Nick Piggin. The problem is following: The size of nodemask_t is greater than the size of long integer, so loading and storing of nodemask_t are not atomic operations. If task->mems_allowed don't intersect with new_mask, such as the first word of the mask is empty and only the first word of new_mask is not empty. When the allocator loads a word of the mask before current->mems_allowed |= new_mask; and then loads another word of the mask after current->mems_allowed = new_mask; the allocator gets an empty nodemask. I make a new patch to fix this problem now. Considering the change of task->mems_allowed is not frequent, so in the new patch, I use variables as a tag to indicate whether task->mems_allowed need be update or not. And before setting the tag, cpuset caches the new mask of every task at somewhere. When the allocator want to access task->mems_allowed, it must check updated-tag first. If the tag is set, the allocator enters the slow path and updates task->mems_allowed. Thanks! Miao -- 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>