On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:

Gregory Price [off-list ref] writes:

quoted

On Wed, Jan 31, 2024 at 02:43:12PM +0800, Huang, Ying wrote:

quoted

Gregory Price [off-list ref] writes:

quoted

 
+static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
+{
+	unsigned int node = current->il_prev;
+
+	if (!current->il_weight || !node_isset(node, policy->nodes)) {
+		node = next_node_in(node, policy->nodes);
+		/* can only happen if nodemask is being rebound */
+		if (node == MAX_NUMNODES)
+			return node;

I feel a little unsafe to read policy->nodes at same time of writing in
rebound.  Is it better to use a seqlock to guarantee its consistency?
It's unnecessary to be a part of this series though.

I think this is handled already? It is definitely an explicit race
condition that is documented elsewhere:

/*
 * mpol_rebind_policy - Migrate a policy to a different set of nodes
 *
 * Per-vma policies are protected by mmap_lock. Allocations using per-task
 * policies are protected by task->mems_allowed_seq to prevent a premature
 * OOM/allocation failure due to parallel nodemask modification.
 */

Thanks for pointing this out!

If we use task->mems_allowed_seq reader side in
weighted_interleave_nodes() we can guarantee the consistency of
policy->nodes.  That may be not deserved, because it's not a big deal to
allocate 1 page in a wrong node.

It makes more sense to do that in
alloc_pages_bulk_array_weighted_interleave(), because a lot of pages may
be allocated there.

That's probably worth just adding now, I'll do it and squash the style
updates into the branch.  Sorry Andrew, I guess 1 last version is
inbound :]

I'll pick up the reviewed tags along the way.

~Gregory