Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to deferred split list

[David Hildenbrand <david@redhat.com>]

On 25.04.24 23:11, 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. But it is possible that
> the folio is fully unmapped and adding it to deferred split list is
> unnecessary.
> 
> For PMD-mapped THPs, that was not really an issue, because removing the
> last PMD mapping in the absence of PTE mappings would not have added the
> folio to the deferred split queue.
> 
> However, for PTE-mapped THPs, which are now more prominent due to mTHP,
> they are always added to the deferred split queue. One side effect
> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
> unintentionally increased, making it look like there are many partially
> mapped folios -- although the whole folio is fully unmapped stepwise.
> 
> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
> folio is unmapped in one go and can avoid being added to deferred split
> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
> -- or where this type of batching is not implemented yet, e.g., migration.
> 
> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
> to tell if the whole folio is unmapped. If the folio is already on
> deferred split list, it will be skipped, too.
> 
> Note: commit 98046944a159 ("mm: huge_memory: add the missing
> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> ---
>   mm/rmap.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a7913a454028..220ad8a83589 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   		 * page of the folio is unmapped and at least one page
>   		 * is still mapped.
>   		 */
> -		if (folio_test_large(folio) && folio_test_anon(folio))
> -			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> -				deferred_split_folio(folio);
> +		if (folio_test_large(folio) && folio_test_anon(folio) &&
> +		    list_empty(&folio->_deferred_list) &&
> +		    ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> +		     (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> +			deferred_split_folio(folio);
>   	}
>   
>   	/*
> 
> base-commit: 66313c66dd90e8711a8b63fc047ddfc69c53636a

Reviewed-by: David Hildenbrand <david@redhat.com>

But maybe we can really improve the code:


diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40dffade..e310b6c4221d7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
  {
         atomic_t *mapped = &folio->_nr_pages_mapped;
         int last, nr = 0, nr_pmdmapped = 0;
+       bool partially_mapped = false;
         enum node_stat_item idx;
  
         __folio_rmap_sanity_checks(folio, page, nr_pages, level);
@@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
                                         nr++;
                         }
                 } while (page++, --nr_pages > 0);
+
+               partially_mapped = nr && atomic_read(mapped);
                 break;
         case RMAP_LEVEL_PMD:
                 atomic_dec(&folio->_large_mapcount);
@@ -1532,6 +1535,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
                                 nr = 0;
                         }
                 }
+               partially_mapped = nr < nr_pmdmapped;
                 break;
         }
  
@@ -1553,9 +1557,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
                  * page of the folio is unmapped and at least one page
                  * is still mapped.
                  */
-               if (folio_test_large(folio) && folio_test_anon(folio))
-                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
-                               deferred_split_folio(folio);
+               if (folio_test_large(folio) && folio_test_anon(folio) &&
+                   list_empty(&folio->_deferred_list) && partially_mapped)
+                       deferred_split_folio(folio);
         }
  
         /*

The compiler should be smart enough to optimize it all -- most likely ;)

-- 
Cheers,

David / dhildenb