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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Mar 19, 2025 at 10:30:20PM +0000, Roman Gushchin wrote:
> Shakeel Butt <shakeel.butt@linux.dev> writes:
> 
> > On Wed, Mar 19, 2025 at 02:41:45PM +0800, Jingxiang Zeng wrote:
> >> From: Zeng Jingxiang <linuszeng@tencent.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@tencent.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.

> I think it deserves a separate config option (if we're really concerned
> about the memory overhead in struct mem_cgroup) or IMO better a
> boot/mount time option.

Yeah, a config option forces distros to enable it :/

I'm hesitant to agree with making it optional in any manner. If you
consider the functionality that is implemented, the overhead should be
fairly minimal. It isn't right now, because page_counter contains a
ton of stuff that isn't applicable to this new user. That overhead is
still paid for unnecessarily by users who _do_ need to enable it.

It seems like a good opportunity to refactor struct page_counter.