Re: [PATCH] mm: workingset: fix vmstat counters for shadow nodes

[Johannes Weiner]

On Thu, Aug 01, 2019 at 04:35:32PM -0700, Roman Gushchin wrote:
> Memcg counters for shadow nodes are broken because the memcg pointer is
> obtained in a wrong way. The following approach is used:
> 	virt_to_page(xa_node)->mem_cgroup
> 
> Since commit 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup
> pointer for slab pages") page->mem_cgroup pointer isn't set for slab pages,
> so memcg_from_slab_page() should be used instead.
> 
> Also I doubt that it ever worked correctly: virt_to_head_page() should be
> used instead of virt_to_page(). Otherwise objects residing on tail pages
> are not accounted, because only the head page contains a valid mem_cgroup
> pointer. That was a case since the introduction of these counters by the
> commit 68d48e6a2df5 ("mm: workingset: add vmstat counter for shadow nodes").

You're right. slub uses order-2 compound pages for radix_tree_node, so
we've been underreporting shadow nodes placed in the three tail pages.

Nice catch.

> Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>