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

[Zi Yan <ziy@nvidia.com>]

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On 11 Apr 2024, at 11:46, David Hildenbrand wrote:

> On 11.04.24 17:32, 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. 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 mapcount before
>> adding a folio to deferred split list.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/rmap.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2608c40dffad..d599a772e282 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   		enum rmap_level level)
>>   {
>>   	atomic_t *mapped = &folio->_nr_pages_mapped;
>> -	int last, nr = 0, nr_pmdmapped = 0;
>> +	int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
>>   	enum node_stat_item idx;
>>    	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   			break;
>>   		}
>>  -		atomic_sub(nr_pages, &folio->_large_mapcount);
>> +		mapcount = atomic_sub_return(nr_pages,
>> +					     &folio->_large_mapcount) + 1;
>
> That becomes a new memory barrier on some archs. Rather just re-read it below. Re-reading should be fine here.

Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped)
below, but to save an atomic op, I chose to read mapcount here.

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Best Regards,
Yan, Zi

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