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Subject: Re: [Resend PATCH 2/6] mm/memcg: remove useless check on
 page->mem_cgroup
To:     Michal Hocko <mhocko@suse.com>
Cc:     akpm@linux-foundation.org, Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <0b5e1ac3-c9c7-35e9-2661-b58430314d0a@linux.alibaba.com>
Date:   Wed, 12 Aug 2020 11:25:53 +0800
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?? 2020/8/11 ????9:56, Michal Hocko д??:
> On Tue 11-08-20 20:54:18, Alex Shi wrote:
>> >From beeac61119ab39b1869c520c0f272fb8bab93765 Mon Sep 17 00:00:00 2001
>> From: Alex Shi <alex.shi@linux.alibaba.com>
>> Date: Wed, 5 Aug 2020 21:02:30 +0800
>> Subject: [PATCH 2/6] memcg: bail out early from swap accounting when memcg is
>>  disabled>>
>> If we disabled memcg by cgroup_disable=memory, the swap charges are
>> still called. Let's return from the funcs earlier.
> They are not, are they? page->memcg will be NULL and so the charge is
> skipped and that will trigger a warning with your current ordering.

Hi Michal,

Thanks for comment! Looks like we both agree the memcg wasn't charged, but funcs
just are called. :)
  
> 
> Let me repeat again. Either you put it first in the series and argue
> that we can bail out early or keep the ordering then this makes sure the
> warning doesn't trigger.
> 

Is the following commit log fine?

Thanks!
Alex

From 999b0fe5fc65865c3b59ff28500d45572a4a9570 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@linux.alibaba.com>
Date: Wed, 5 Aug 2020 21:02:30 +0800
Subject: [PATCH 2/6] mm/memcg: bail out early from swap accounting when memcg
 is disabled

If we disabled memcg by cgroup_disable=memory, page->memcg will be NULL
and so the charge is skipped and that will trigger a warning like below.
Let's return from the funcs earlier.

 ---[ end trace f1f34bfc3b32ed2f ]---
 anon flags:0x5005b48008000d(locked|uptodate|dirty|swapbacked)
 raw: 005005b48008000d dead000000000100 dead000000000122 ffff8897c7c76ad1
 raw: 0000000000000022 0000000000000000 0000000200000000 0000000000000000
 page dumped because: VM_WARN_ON_ONCE_PAGE(!memcg)

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: cgroups@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/memcontrol.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 299382fc55a9..419cf565f40b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7098,6 +7098,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 	VM_BUG_ON_PAGE(page_count(page), page);
 
+	if (mem_cgroup_disabled())
+		return;
+
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return;
 
@@ -7163,6 +7166,9 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
 	struct mem_cgroup *memcg;
 	unsigned short oldid;
 
+	if (mem_cgroup_disabled())
+		return 0;
+
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return 0;
 
-- 
1.8.3.1