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

[Roman Gushchin]

Shakeel Butt <shakeel.butt@xxxxxxxxx> writes:

> On Wed, Mar 19, 2025 at 02:41:45PM +0800, Jingxiang Zeng wrote:
>> From: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>
>> 
>> 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@xxxxxxxxxxx>
>> ---
>>  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.

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.

Thanks!