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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Aug 18, 2025 at 05:36:53PM -0700, Boris Burkov wrote:
> +++ b/mm/filemap.c
> @@ -960,15 +960,19 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
>  {
>  	void *shadow = NULL;
>  	int ret;
> +	bool charge_mem_cgroup = !test_bit(AS_UNCHARGED, &mapping->flags);
>  
> -	ret = mem_cgroup_charge(folio, NULL, gfp);
> -	if (ret)
> -		return ret;
> +	if (charge_mem_cgroup) {
> +		ret = mem_cgroup_charge(folio, NULL, gfp);
> +		if (ret)
> +			return ret;
> +	}
>  
>  	__folio_set_locked(folio);
>  	ret = __filemap_add_folio(mapping, folio, index, gfp, &shadow);
>  	if (unlikely(ret)) {
> -		mem_cgroup_uncharge(folio);
> +		if (charge_mem_cgroup)
> +			mem_cgroup_uncharge(folio);
>  		__folio_clear_locked(folio);

This is unnecessarily complex; mem_cgroup_uncharge() is a no-op if
the folio is not charged.  Sure, it's a wasted function call, but
this is a rare error path, so minimising size is more important than
minimising time.

So you can drop the 'bool' as well:

+	if (!test_bit(AS_UNCHARGED, &mapping->flags)) {