On 17.07.2019 20:45, Yang Shi wrote:
> Shakeel Butt reported premature oom on kernel with
> "cgroup_disable=memory" since mem_cgroup_is_root() returns false even
> though memcg is actually NULL. The drop_caches is also broken.
>
> It is because commit aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab()
> calls in shrink_node()") removed the !memcg check before
> !mem_cgroup_is_root(). And, surprisingly root memcg is allocated even
> though memory cgroup is disabled by kernel boot parameter.
>
> Add mem_cgroup_disabled() check to make reclaimer work as expected.
>
> Fixes: aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab() calls in shrink_node()")
> Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Qian Cai <cai@xxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx 4.19+
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Surprise really.
We have mem_cgroup as not early inited, so all of these boundary
cases and checks has to be supported. But it looks like it's not
possible to avoid that in any way.
> ---
> mm/vmscan.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f8e3dcd..c10dc02 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -684,7 +684,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> unsigned long ret, freed = 0;
> struct shrinker *shrinker;
>
> - if (!mem_cgroup_is_root(memcg))
> + /*
> + * The root memcg might be allocated even though memcg is disabled
> + * via "cgroup_disable=memory" boot parameter. This could make
> + * mem_cgroup_is_root() return false, then just run memcg slab
> + * shrink, but skip global shrink. This may result in premature
> + * oom.
> + */
> + if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
> return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>
> if (!down_read_trylock(&shrinker_rwsem))
>
