linux-kernel - Re: [v2 PATCH 3/9] mm: vmscan: guarantee shrinker_slab_memcg() sees valid shrinker

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Date:   Tue, 15 Dec 2020 13:38:02 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     guro@...com, ktkhai@...tuozzo.com, shakeelb@...gle.com,
        david@...morbit.com, mhocko@...e.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 3/9] mm: vmscan: guarantee shrinker_slab_memcg() sees
 valid shrinker_maps for online memcg

On Mon, Dec 14, 2020 at 02:37:16PM -0800, Yang Shi wrote:
> The shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag
> in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that we will see
> memcg->nodeinfo[nid]->shrinker_maps != NULL.  This may occur because of processor reordering
> on !x86.
> 
> This seems like the below case:
> 
>            CPU A          CPU B
> store shrinker_map      load CSS_ONLINE
> store CSS_ONLINE        load shrinker_map
> 
> So the memory ordering could be guaranteed by smp_wmb()/smp_rmb() pair.
> 
> The memory barriers pair will guarantee the ordering between shrinker_deferred and CSS_ONLINE
> for the following patches as well.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>

As per previous feedback, please move the misplaced shrinker
allocation callback from .css_online to .css_alloc. This will get you
the necessary ordering guarantees from the cgroup core code.

Thanks