Hi Qi.

On Wed, Dec 17, 2025 at 03:27:51PM +0800, Qi Zheng <qi.zheng@linux.dev> wrote:

> @@ -5200,22 +5238,27 @@ int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry)
>  	unsigned int nr_pages = folio_nr_pages(folio);
>  	struct page_counter *counter;
>  	struct mem_cgroup *memcg;
> +	struct obj_cgroup *objcg;
>  
>  	if (do_memsw_account())
>  		return 0;
>  
> -	memcg = folio_memcg(folio);
> -
> -	VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
> -	if (!memcg)
> +	objcg = folio_objcg(folio);
> +	VM_WARN_ON_ONCE_FOLIO(!objcg, folio);
> +	if (!objcg)
>  		return 0;
>  
> +	rcu_read_lock();
> +	memcg = obj_cgroup_memcg(objcg);
>  	if (!entry.val) {
>  		memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
> +		rcu_read_unlock();
>  		return 0;
>  	}
>  
>  	memcg = mem_cgroup_id_get_online(memcg);
> +	/* memcg is pined by memcg ID. */
> +	rcu_read_unlock();
>  
>  	if (!mem_cgroup_is_root(memcg) &&
>  	    !page_counter_try_charge(&memcg->swap, nr_pages, &counter)) {

Later there is:
	swap_cgroup_record(folio, mem_cgroup_id(memcg), entry);

As per the comment memcg remains pinned by the ID which is associated
with a swap slot, i.e. theoretically time unbound (shmem).
(This was actually brought up by Yosry in stats subthread [1])

I think that should be tackled too to eliminate the problem completely.

As I look at the code, these memcg IDs (private [2]) could be converted
to objcg IDs so that reparenting applies also to folios that are
currently swapped out. (Or convert to swap_cgroup_ctrl from the vector
of IDs to a vector of objcg pointers, depending on space.)

Thanks,
Michal

[1] https://lore.kernel.org/r/ebdhvcwygvnfejai5azhg3sjudsjorwmlcvmzadpkhexoeq3tb@5gj5y2exdhpn
[2] https://lore.kernel.org/r/20251225232116.294540-1-shakeel.butt@linux.dev