linux-kernel - Re: [RFC 2/5] memcontrol: add boot option to enable memsw account on dfl

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Message-ID: <ipskzxjtm656f5srrp42uxemh5e4jdwzsyj2isqlldfaokiyoo@ly4gfvldjc2p>
Date: Thu, 20 Mar 2025 08:33:09 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, 
	Jingxiang Zeng <linuszeng@...cent.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, mhocko@...nel.org, 
	muchun.song@...ux.dev, kasong@...cent.com
Subject: Re: [RFC 2/5] memcontrol: add boot option to enable memsw account on
 dfl

On Thu, Mar 20, 2025 at 10:28:46AM -0400, Johannes Weiner wrote:
> On Wed, Mar 19, 2025 at 10:30:20PM +0000, Roman Gushchin wrote:
> > Shakeel Butt <shakeel.butt@...ux.dev> writes:
> > 
> > > On Wed, Mar 19, 2025 at 02:41:45PM +0800, Jingxiang Zeng wrote:
> > >> From: Zeng Jingxiang <linuszeng@...cent.com>
> > >> 
> > >> Added cgroup.memsw_account_on_dfl startup parameter, which
> > >> is off by default. When enabled in cgroupv2 mode, the memory
> > >> accounting mode of swap will be reverted to cgroupv1 mode.
> > >> 
> > >> Signed-off-by: Zeng Jingxiang <linuszeng@...cent.com>
> > >> ---
> > >>  include/linux/memcontrol.h |  4 +++-
> > >>  mm/memcontrol.c            | 11 +++++++++++
> > >>  2 files changed, 14 insertions(+), 1 deletion(-)
> > >> 
> > >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > >> index dcb087ee6e8d..96f2fad1c351 100644
> > >> --- a/include/linux/memcontrol.h
> > >> +++ b/include/linux/memcontrol.h
> > >> @@ -62,10 +62,12 @@ struct mem_cgroup_reclaim_cookie {
> > >>  
> > >>  #ifdef CONFIG_MEMCG
> > >>  
> > >> +DECLARE_STATIC_KEY_FALSE(memsw_account_on_dfl);
> > >>  /* Whether enable memory+swap account in cgroupv2 */
> > >>  static inline bool do_memsw_account_on_dfl(void)
> > >>  {
> > >> -	return IS_ENABLED(CONFIG_MEMSW_ACCOUNT_ON_DFL);
> > >> +	return IS_ENABLED(CONFIG_MEMSW_ACCOUNT_ON_DFL)
> > >> +				|| static_branch_unlikely(&memsw_account_on_dfl);
> > >
> > > Why || in above condition? Shouldn't it be && ?
> > >
> > >>  }
> > >>  
> > >>  #define MEM_CGROUP_ID_SHIFT	16
> > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > >> index 768d6b15dbfa..c1171fb2bfd6 100644
> > >> --- a/mm/memcontrol.c
> > >> +++ b/mm/memcontrol.c
> > >> @@ -5478,3 +5478,14 @@ static int __init mem_cgroup_swap_init(void)
> > >>  subsys_initcall(mem_cgroup_swap_init);
> > >>  
> > >>  #endif /* CONFIG_SWAP */
> > >> +
> > >> +DEFINE_STATIC_KEY_FALSE(memsw_account_on_dfl);
> > >> +static int __init memsw_account_on_dfl_setup(char *s)
> > >> +{
> > >> +	if (!strcmp(s, "1"))
> > >> +		static_branch_enable(&memsw_account_on_dfl);
> > >> +	else if (!strcmp(s, "0"))
> > >> +		static_branch_disable(&memsw_account_on_dfl);
> > >> +	return 1;
> > >> +}
> > >> +__setup("cgroup.memsw_account_on_dfl=", memsw_account_on_dfl_setup);
> > >
> > > Please keep the above in memcontrol-v1.c
> > 
> > Hm, I'm not sure about this. This feature might be actually useful with
> > cgroup v2, as some companies are dependent on the old cgroup v1
> > semantics here but otherwise would prefer to move to v2.
> > In other words, I see it as a cgroup v2 feature, not as a cgroup v1.
> > So there is no reason to move it into the cgroup v1 code.
> 
> Agreed. Let's think of this proposal as making memsw tracking and
> control a full-fledged v2 feature.
> 

Sounds good. However I want us to discuss and decide the semantics of
memsw from scratch rather than adopting v1 semantics. Particularly I
don't like the failure of setting memsw limit on v1. Also we should
discuss how memsw and swap limits would interact and what would be the
appropriate default.