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

[Zi Yan <ziy@nvidia.com>]

[-- Attachment #1: Type: text/plain, Size: 5439 bytes --]

On 26 Apr 2024, at 4:19, David Hildenbrand wrote:

> 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 ;)

Sure. Let me send a new one using your changes with folio_test_large(folio) dropped
like you said. Yours is easier to understand. Thank you for helping.

--
Best Regards,
Yan, Zi

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]