Re: [PATCH 1/4] memcg: keep swap charging under RCU protection

[Shakeel Butt]

On Fri, Sep 18, 2026 at 05:18:40PM +0800, Bingfang Guo via B4 Relay wrote:
> From: Bingfang Guo <bingfangguo@xxxxxxxxxxx>
> 
> This is a preparatory work for unbinding memcgid from memcg. No
> functional change.
> 
> The swap charging path currently drops its RCU read lock after acquiring
> a private ID reference. This is safe because the ID reference pins the
> memcg's CSS.
> 
> Moving private ID references to objcgs will remove that lifetime
> guarantee. Keep the RCU read lock held while accessing the memcg for
> counter charging, statistics and failure handling. (This matches what
> __memcg1_swapout() already does.).
> 
> Save the private ID before dropping the RCU read lock, and use the saved
> value when recording the swap entry. The swap cluster locking remains
> outside the RCU read-side critical section.
> 
> Signed-off-by: Bingfang Guo <bingfangguo@xxxxxxxxxxx>
> ---
>  mm/memcontrol.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 791e536efaebe..72522ec827c9a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5954,6 +5954,7 @@ int __mem_cgroup_try_charge_swap(struct folio *folio)
>  	struct page_counter *counter;
>  	struct mem_cgroup *memcg;
>  	struct obj_cgroup *objcg;
> +	unsigned short private_id;
>  
>  	if (do_memsw_account())
>  		return 0;
> @@ -5973,20 +5974,21 @@ int __mem_cgroup_try_charge_swap(struct folio *folio)
>  

If there is one more spin of this patch, I think scoped_guard(rcu) would be more
readable here, so please use that here.

>  	memcg = mem_cgroup_private_id_get_online(memcg, nr_pages);
>  	/* memcg is pined by memcg ID. */
> -	rcu_read_unlock();
> +	private_id = mem_cgroup_private_id(memcg);
>  
>  	if (!mem_cgroup_is_root(memcg) &&
>  	    !page_counter_try_charge(&memcg->swap, nr_pages, &counter)) {
>  		memcg_memory_event(memcg, MEMCG_SWAP_MAX);
>  		memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
>  		mem_cgroup_private_id_put(memcg, nr_pages);
> +		rcu_read_unlock();
>  		return -ENOMEM;
>  	}
>  	mod_memcg_state(memcg, MEMCG_SWAP, nr_pages);
> +	rcu_read_unlock();
>  
>  	ci = swap_cluster_get_and_lock(folio);
> -	__swap_cgroup_set(ci, swp_cluster_offset(folio->swap), nr_pages,
> -			  mem_cgroup_private_id(memcg));
> +	__swap_cgroup_set(ci, swp_cluster_offset(folio->swap), nr_pages, private_id);
>  	swap_cluster_unlock(ci);
>  
>  	return 0;
> 
> -- 
> 2.43.7
> 
>