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

[David Hildenbrand <david@redhat.com>]

On 12.04.24 23:06, Zi Yan wrote:
> On 12 Apr 2024, at 15:32, David Hildenbrand wrote:
> 
>> On 12.04.24 16:35, Zi Yan wrote:
>>> 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.
>>
>> Some points:
>>
>> (1) I suggest reading about atomic get/set vs. atomic RMW vs. atomic
>> RMW that return a value -- and how they interact with memory barriers.
>> Further, how relaxed variants are only optimized on some architectures.
>>
>> atomic_read() is usually READ_ONCE(), which is just an "ordinary" memory
>> access that should not be refetched. Usually cheaper than most other stuff
>> that involves atomics.
> 
> I should have checked the actual implementation instead of being fooled
> by the name. Will read about it. Thanks.
> 
>>
>> (2) We can either use folio_large_mapcount() == 0 or !atomic_read(mapped)
>> to figure out if the folio is now completely unmapped.
>>
>> (3) There is one fundamental issue: if we are not batch-unmapping the whole
>> thing, we will still add the folios to the deferred split queue. Migration
>> would still do that, or if there are multiple VMAs covering a folio.
>>
>> (4) We should really avoid making common operations slower only to make
>> some unreliable stats less unreliable.
>>
>>
>> We should likely do something like the following, which might even be a bit
>> faster in some cases because we avoid a function call in case we unmap
>> individual PTEs by checking _deferred_list ahead of time
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2608c40dffad..356598b3dc3c 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) &&
>> +                   (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) &&
>> +                   atomic_read(mapped) &&
>> +                   data_race(list_empty(&folio->_deferred_list)))
> 
> data_race() might not be needed, as Ryan pointed out[1]

Right, I keep getting confused by that. Likely we should add data_race() 
only if we get actual reports.

-- 
Cheers,

David / dhildenb