On Tue, Oct 29, 2019 at 04:47:53PM -0700, Shakeel Butt wrote:
> Since commit 1ba6fc9af35b ("mm: vmscan: do not share cgroup iteration
> between reclaimers"), the memcg reclaim does not bail out earlier based
> on sc->nr_reclaimed and will traverse all the nodes. All the reclaimable
> pages of the memcg on all the nodes will be scanned relative to the
> reclaim priority. So, there is no need to maintain state regarding which
> node to start the memcg reclaim from. Also KCSAN complains data races in
> the code maintaining the state.
>
> This patch effectively reverts the commit 889976dbcb12 ("memcg: reclaim
> memory from nodes in round-robin order") and the commit 453a9bf347f1
> ("memcg: fix numa scan information update to be triggered by memory
> event").
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Reported-by: <syzbot+13f93c99c06988391efe@xxxxxxxxxxxxxxxxxxxxxxxxx>
Excellent, thanks Shakeel!
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Just a request on this bit:
> @@ -3360,16 +3358,9 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> .may_unmap = 1,
> .may_swap = may_swap,
> };
> + struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
>
> set_task_reclaim_state(current, &sc.reclaim_state);
> - /*
> - * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
> - * take care of from where we get pages. So the node where we start the
> - * scan does not need to be the current node.
> - */
> - nid = mem_cgroup_select_victim_node(memcg);
> -
> - zonelist = &NODE_DATA(nid)->node_zonelists[ZONELIST_FALLBACK];
This works, but it *is* somewhat fragile if we decide to add bail-out
conditions to reclaim again. And some numa nodes receiving slightly
less pressure than others could be quite tricky to debug.
Can we add a comment here that points out the assumption that the
zonelist walk is comprehensive, and that all nodes receive equal
reclaim pressure?
Also, I think we should use sc.gfp_mask & ~__GFP_THISNODE, so that
allocations with a physical node preference still do node-agnostic
reclaim for the purpose of cgroup accounting.
