Re: [PATCH] mm/memcg: remove useless check on page->mem_cgroup

[Michal Hocko <mhocko@suse.com>]

On Wed 05-08-20 21:02:30, Alex Shi wrote:
> 
> 
> 在 2020/8/5 下午8:28, Alex Shi 写道:
> > The last patch has a problem on define. this version could fix it.
> > 
> > BTW, I see some !memcg happens when MEMCG compilered but disabled by cgroup_disable
> > 
> > 
> > [   94.657666] ---[ end trace f1f34bfc3b32ed2f ]---
> > [   95.138995] anon flags: 0x5005b48008000d(locked|uptodate|dirty|swapbacked)
> > [   95.146220] raw: 005005b48008000d dead000000000100 dead000000000122 ffff8897c7c76ad1
> > [   95.154549] raw: 0000000000000022 0000000000000000 0000000200000000 0000000000000000
> > [   95.162876] page dumped because: VM_WARN_ON_ONCE_PAGE(!memcg)
> > 
> > 
> 
> The following patch may helpful.
> 
> >From 8bfb26a2e37e08dc61d20212bcfa5812a367ba94 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@linux.alibaba.com>
> Date: Wed, 5 Aug 2020 20:32:12 +0800
> Subject: [PATCH] mm/memcg: don't try charge swap if memcg disabled
> 
> If we disabled memcg by cgroup_disable=memory, the swap charges are
> still called. Let's return from the funcs earlier and keep WARN_ON
> monitor.

Do I get it right that this is on top of your patch to remove the memcg
check or a preparatory work? Both are good but it would be better to
call that out specifically for clarity (along with the warning if that
is a follow up fix).

> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.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

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index cb07a48d53aa..65f2b42d25af 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7163,6 +7163,9 @@ 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;
>  
> @@ -7228,6 +7231,9 @@ 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

-- 
Michal Hocko
SUSE Labs