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Subject: Re: [PATCH v7 15/17] mm: Generalize shrink_slab() calls in
 shrink_node()
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        stummala@codeaurora.org, gregkh@linuxfoundation.org,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Roman Gushchin <guro@fb.com>, mka@chromium.org,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Chris Wilson <chris@chris-wilson.co.uk>, longman@redhat.com,
        Minchan Kim <minchan@kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>, jbacik@fb.com,
        Guenter Roeck <linux@roeck-us.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Matthew Wilcox <willy@infradead.org>, lirongqing@baidu.com,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Sat, 9 Jun 2018 11:46:51 +0300
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Hi, Shakeel.

On 08.06.2018 22:21, Shakeel Butt wrote:
> On Tue, May 22, 2018 at 3:09 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>> From: Vladimir Davydov <vdavydov.dev@gmail.com>
>>
>> The patch makes shrink_slab() be called for root_mem_cgroup
>> in the same way as it's called for the rest of cgroups.
>> This simplifies the logic and improves the readability.
>>
>> Signed-off-by: Vladimir Davydov <vdavydov.dev@gmail.com>
>> ktkhai: Description written.
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>>  mm/vmscan.c |   21 ++++++---------------
>>  1 file changed, 6 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index f26ca1e00efb..6dbc659db120 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -628,10 +628,8 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>   * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
>>   * unaware shrinkers will receive a node id of 0 instead.
>>   *
>> - * @memcg specifies the memory cgroup to target. If it is not NULL,
>> - * only shrinkers with SHRINKER_MEMCG_AWARE set will be called to scan
>> - * objects from the memory cgroup specified. Otherwise, only unaware
>> - * shrinkers are called.
>> + * @memcg specifies the memory cgroup to target. Unaware shrinkers
>> + * are called only if it is the root cgroup.
>>   *
>>   * @priority is sc->priority, we take the number of objects and >> by priority
>>   * in order to get the scan target.
>> @@ -645,7 +643,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>         struct shrinker *shrinker;
>>         unsigned long freed = 0;
>>
> 
> Shouldn't there be a VM_BUG_ON(!memcg) here?

memcg can be NULL here in case of memory controller is disabled at boot parameters
and in case of it's a global reclaim. So, such the check we can't add here.

Thanks,
Kirill