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

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Message-ID: <20190805194341.GA6260@cmpxchg.org>
Date:   Mon, 5 Aug 2019 15:43:41 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm: workingset: fix vmstat counters for shadow nodes

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@...com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>

Acked-by: Johannes Weiner <hannes@...xchg.org>