Re: [External] Re: [PATCH 4/5] mm: memcg/slab: Fix root memcg vmstats

[Roman Gushchin]

On Wed, Oct 28, 2020 at 10:56:20AM +0800, Muchun Song wrote:
> On Wed, Oct 28, 2020 at 2:48 AM Roman Gushchin <guro@xxxxxx> wrote:
> >
> > On Tue, Oct 27, 2020 at 04:02:55PM +0800, Muchun Song wrote:
> > > If we reparent the slab objects to the root memcg, when we free
> > > the slab object, we need to update the per-memcg vmstats to keep
> > > it correct for the root memcg. Now this at least affects the vmstat
> > > of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
> > > size is smaller than the PAGE_SIZE.
> > >
> > > Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > Can you, please, drop this patch for now?
> >
> > I'm working on a bigger cleanup related to the handling of the root memory
> > cgroup (I sent a link earlier in this thread), which already does a similar change.
> > There are several issues like this one, so it will be nice to fix them all at once.
> 
> I have read the patch of https://lkml.org/lkml/2020/10/14/869. You
> mean this patch
> fixes this issue? It chooses to uncharge the root memcg. But here we may need to
> uncharge the root memcg to keep root vmstats correct. If we do not do
> this, we can
> see the wrong vmstats via root memory.stat(e.g. NR_KERNEL_STACK_KB).

I pointed at a different patch in the same thread (it looks like you read the first one):
https://lkml.org/lkml/2020/10/21/612

It contained the following part:

@@ -868,7 +860,7 @@ void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
 	memcg = mem_cgroup_from_obj(p);
 
 	/* Untracked pages have no memcg, no lruvec. Update only the node */
-	if (!memcg || memcg == root_mem_cgroup) {
+	if (!memcg) {
 		__mod_node_page_state(pgdat, idx, val);
 	} else {
 		lruvec = mem_cgroup_lruvec(memcg, pgdat);

So it's exactly what your patch does.

Thanks!