Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

[David Hildenbrand <david@redhat.com>]

On 11.04.24 17:32, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> In __folio_remove_rmap(), a large folio is added to deferred split list
> if any page in a folio loses its final mapping. It is possible that
> the folio is unmapped fully, but it is unnecessary to add the folio
> to deferred split list at all. Fix it by checking folio mapcount before
> adding a folio to deferred split list.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   mm/rmap.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2608c40dffad..d599a772e282 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   		enum rmap_level level)
>   {
>   	atomic_t *mapped = &folio->_nr_pages_mapped;
> -	int last, nr = 0, nr_pmdmapped = 0;
> +	int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
>   	enum node_stat_item idx;
>   
>   	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   			break;
>   		}
>   
> -		atomic_sub(nr_pages, &folio->_large_mapcount);
> +		mapcount = atomic_sub_return(nr_pages,
> +					     &folio->_large_mapcount) + 1;

That becomes a new memory barrier on some archs. Rather just re-read it 
below. Re-reading should be fine here.

>   		do {
>   			last = atomic_add_negative(-1, &page->_mapcount);
>   			if (last) {
> @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   		 * is still mapped.
>   		 */
>   		if (folio_test_large(folio) && folio_test_anon(folio))
> -			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> +			if ((level == RMAP_LEVEL_PTE &&
> +			     mapcount != 0) ||
> +			    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))
>   				deferred_split_folio(folio);
>   	}

But I do wonder if we really care? Usually the folio will simply get 
freed afterwards, where we simply remove it from the list.

If it's pinned, we won't be able to free or reclaim, but it's rather a 
corner case ...

Is it really worth the added code? Not convinced.

-- 
Cheers,

David / dhildenb