Re: [PATCH v3 1/4] mm/filemap: add AS_UNCHARGED

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Aug 21, 2025 at 12:06:42AM +0200, Klara Modin wrote:
[...]
> 
> The following diff resolves the issue for me:
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index fae105a9cb46..c70e789201fc 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -809,7 +809,7 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  
>  static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
>  {
> -	if (mem_cgroup_disabled())
> +	if (mem_cgroup_disabled() || !memcg)
>  		return 0;
>  
>  	return memcg->id.id;
> 
> However, it's mentioned in folio_memcg() that it can return NULL so this
> might be an existing bug which this patch just makes more obvious.
> 
> There's also workingset_eviction() which instead gets the memcg from
> lruvec. Doing that in lru_gen_eviction() also resolves the issue for me:
> 
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 68a76a91111f..e805eadf0ec7 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -243,6 +243,7 @@ static void *lru_gen_eviction(struct folio *folio)
>  	int tier = lru_tier_from_refs(refs, workingset);
>  	struct mem_cgroup *memcg = folio_memcg(folio);
>  	struct pglist_data *pgdat = folio_pgdat(folio);
> +	int memcgid;
>  
>  	BUILD_BUG_ON(LRU_GEN_WIDTH + LRU_REFS_WIDTH > BITS_PER_LONG - EVICTION_SHIFT);
>  
> @@ -254,7 +255,9 @@ static void *lru_gen_eviction(struct folio *folio)
>  	hist = lru_hist_from_seq(min_seq);
>  	atomic_long_add(delta, &lrugen->evicted[hist][type][tier]);
>  
> -	return pack_shadow(mem_cgroup_id(memcg), pgdat, token, workingset);
> +	memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
> +
> +	return pack_shadow(memcgid, pgdat, token, workingset);
>  }
>  
>  /*
> 
> I don't really know what I'm doing here, though.

Thanks a lot for the report and I think we will prefer your second
approach which makes lru_gen_eviction() similar to
workingset_eviction(). Can you please send a formal patch and Boris can
add that patch in his series?