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

[Zi Yan <ziy@nvidia.com>]

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On 25 Apr 2024, at 21:45, Barry Song wrote:

> On Fri, Apr 26, 2024 at 5:11 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. 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);
>
> Hi Zi Yan,
> in case a mTHP is mapped by two processed (forked but not CoW yet), if we
> unmap the whole folio by pte level in one process only, are we still adding this
> folio into deferred list?

No. Because the mTHP is still fully mapped by the other process. In terms of code,
nr will be 0 in that case and this if condition is skipped. nr is only increased
from 0 when one of the subpages in the mTHP has no mapping, namely page->_mapcount
becomes negative and last is true in the case RMAP_LEVEL_PTE.


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Best Regards,
Yan, Zi

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