Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list

[Zi Yan <ziy@nvidia.com>]

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On 24 Apr 2024, at 18:32, Yang Shi wrote:

> On Wed, Apr 24, 2024 at 2:10 PM 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
>> 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.
>>
>> 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 everything. A fully unmapped PTE-mapped order-9 THP was also 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.
>
> Shall you mention the miscounting for mTHP too? There is another patch
> series adding the counter support for mTHP.

OK, will add it.
>
>>
>> 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>
>> ---
>>  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) &&
>
> Do we really need this check? deferred_split_folio() does the same
> check too. Bailing out earlier sounds ok too, but there may not be too
> much gain.

Sure, I can remove it.

>
>> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
>> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
>
> IIUC, this line is used to cover the case which has both partial
> PTE-mapping and PMD-mapping, then PMD mapping is unmapped fully. IIRC
> this case was not handled correctly before, the THP actually skipped
> deferred split queue. If so please add some description in the commit
> log.

It is properly handled before, since the original code is
(level == RMAP_LEVEL_PTE || nr < nr_pmdmapped), meaning
if either level is RMAP_LEVEL_PTE or
(level == RMAP_LEVEL_PMD && nr < nr_pmdmapped), the folio
is added to the deferred split list. So only level == RMAP_LEVEL_PTE
part of logic needs to be fixed.

>
> Otherwise the patch looks good to me. Reviewed-by: Yang Shi
> <shy828301@gmail.com>
>

Thanks.
>> +                       deferred_split_folio(folio);
>>         }
>>
>>         /*
>>
>> base-commit: 2541ee5668b019c486dd3e815114130e35c1495d
>> --
>> 2.43.0
>>


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Best Regards,
Yan, Zi

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