Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list

[David Hildenbrand <david@redhat.com>]

On 25.04.24 05:45, Lance Yang wrote:
> Hey Zi,
> 
> On Thu, Apr 25, 2024 at 6:46 AM Zi Yan <zi.yan@sent.com> 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
> 
> Agreed. If a folio is fully unmapped, then that's unnecessary to add
> to the deferred split list.
> 
>> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
>> before adding a folio to deferred split list. If the folio is already
>> on the deferred split list, it will be skipped. This issue applies to
>> both PTE-mapped THP and mTHP.
>>
>> 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().
>> However, this miscount was present even earlier due to implementation,
>> since PTEs are unmapped individually and first PTE unmapping adds the THP
>> into the deferred split list.
>>
>> With commit b06dc281aa99 ("mm/rmap: introduce
>> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
>> folios in one shot without causing the miscount, hence this patch.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Yang Shi <shy828301@gmail.com>
>> ---
>>   mm/rmap.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index a7913a454028..2809348add7b 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1553,9 +1553,10 @@ 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) &&
>> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
>> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> 
> Perhaps we only need to check the mapcount?
> 
> IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here,
> then folio_mapcount() will return 0.

See discussion on v1. folio_large_mapcount() would achieve the same 
without another folio_test_large() check, but in the context of this 
patch it doesn't really matter.

-- 
Cheers,

David / dhildenb