Re: [PATCH] mm/fadvise: avoid remote LRU drain for mapped folio failures

[David Hildenbrand (Arm)]

> -long mapping_evict_folio(struct address_space *mapping, struct folio *folio)
> +static long __mapping_evict_folio(struct address_space *mapping,
> +				  struct folio *folio, bool *lru_refs)
>  {
> +	if (lru_refs)
> +		*lru_refs = false;
>  	/* The page may have been truncated before it was locked */
>  	if (!mapping)
>  		return 0;
> @@ -331,14 +323,38 @@ long mapping_evict_folio(struct address_space *mapping, struct folio *folio)
>  		return 0;
>  	/* The refcount will be elevated if any page in the folio is mapped */
>  	if (folio_ref_count(folio) >
> -			folio_nr_pages(folio) + folio_has_private(folio) + 1)
> +			folio_nr_pages(folio) + folio_has_private(folio) + 1) {
> +		/*
> +		 * A remote LRU drain can only help with extra references on
> +		 * otherwise evictable folios.  Mapped folios also have an
> +		 * elevated refcount, but draining LRU caches cannot unmap them.
> +		 */
> +		if (lru_refs && !folio_mapped(folio))
> +			*lru_refs = true;

How can you conclude that it's a LRU refs? Could also just be something trivial
like a remaining GUP reference, or am I wrong?

-- 
Cheers,

David