Re: [PATCH -next] memcg: move mem_cgroup_swappiness to memcontrol.h

[Ridong Chen <ridong.chen@linux.dev>]

On 7/10/2026 7:12 PM, Ridong wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> The per-memcg swappiness knob is v1-only; v2 always uses global
> vm_swappiness and ignores the per-cgroup field.
> 
> Guard memcg->swappiness with CONFIG_MEMCG_V1, and move the helper
> to memcontrol.h where it belongs.
> 
> No functional change for v1; v2-only kernels drop the unused field.
> 
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>  include/linux/memcontrol.h | 25 +++++++++++++++++++++++--
>  include/linux/swap.h       | 19 -------------------
>  mm/memcontrol.c            |  3 +--
>  3 files changed, 24 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e1f46a0016fc..f59614956f96 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -239,8 +239,6 @@ struct mem_cgroup {
>  	 */
>  	bool oom_group;
>  
> -	int swappiness;
> -
>  	/* memory.events and memory.events.local */
>  	struct cgroup_file events_file;
>  	struct cgroup_file events_local_file;
> @@ -318,6 +316,9 @@ struct mem_cgroup {
>  	/* List of events which userspace want to receive */
>  	struct list_head event_list;
>  	spinlock_t event_list_lock;
> +
> +	int swappiness;
> +
>  #endif /* CONFIG_MEMCG_V1 */
>  
>  	struct mem_cgroup_per_node *nodeinfo[];
> @@ -365,6 +366,9 @@ enum objext_flags {
>  
>  #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
>  
> +/* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
> +extern int vm_swappiness;
> +
>  #ifdef CONFIG_MEMCG
>  /*
>   * After the initialization objcg->memcg is always pointing at
> @@ -1440,6 +1444,23 @@ static inline void mem_cgroup_flush_workqueue(void) { }
>  static inline int mem_cgroup_init(void) { return 0; }
>  #endif /* CONFIG_MEMCG */
>  
> +static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> +{
> +#ifdef CONFIG_MEMCG_V1
> +	/* Cgroup2 doesn't have per-cgroup swappiness */
> +	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +		return READ_ONCE(vm_swappiness);
> +
> +	/* root ? */
> +	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
> +		return READ_ONCE(vm_swappiness);
> +
> +	return READ_ONCE(memcg->swappiness);
> +#else
> +	return READ_ONCE(vm_swappiness);
> +#endif
> +}
> +
>  /*
>   * Extended information for slab objects stored as an array in page->memcg_data
>   * if MEMCG_DATA_OBJEXTS is set.
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 3f31b6a56788..f27e73f29195 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -366,7 +366,6 @@ extern unsigned long mem_cgroup_shrink_node(struct mem_cgroup *mem,
>  						pg_data_t *pgdat,
>  						unsigned long *nr_scanned);
>  extern unsigned long shrink_all_memory(unsigned long nr_pages);
> -extern int vm_swappiness;
>  long remove_mapping(struct address_space *mapping, struct folio *folio);
>  
>  #if defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
> @@ -530,25 +529,7 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
>  }
>  #endif /* CONFIG_SWAP */
>  #ifdef CONFIG_MEMCG
> -static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> -{
> -	/* Cgroup2 doesn't have per-cgroup swappiness */
> -	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> -		return READ_ONCE(vm_swappiness);
> -
> -	/* root ? */
> -	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
> -		return READ_ONCE(vm_swappiness);
> -
> -	return READ_ONCE(memcg->swappiness);
> -}
> -
>  void lru_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent, int nid);
> -#else
> -static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
> -{
> -	return READ_ONCE(vm_swappiness);
> -}
>  #endif
>  
>  #if defined(CONFIG_SWAP) && defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 938f190a98fe..2d659b76cd77 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4174,11 +4174,10 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>  #endif
>  	page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
>  	if (parent) {
> -		WRITE_ONCE(memcg->swappiness, mem_cgroup_swappiness(parent));
> -
>  		page_counter_init(&memcg->memory, &parent->memory, memcg_on_dfl);
>  		page_counter_init(&memcg->swap, &parent->swap, false);
>  #ifdef CONFIG_MEMCG_V1
> +		WRITE_ONCE(memcg->swappiness, mem_cgroup_swappiness(parent));
>  		memcg->memory.track_failcnt = !memcg_on_dfl;
>  		WRITE_ONCE(memcg->oom_kill_disable, READ_ONCE(parent->oom_kill_disable));
>  		page_counter_init(&memcg->kmem, &parent->kmem, false);

I have resent this patch [1], as I found another issue related to it.

[1]
https://lore.kernel.org/linux-mm/20260711091157.306070-1-ridong.chen@linux.dev/

-- 
Best regards,
Ridong