Re: [PATCH v2 2/5] mm: Allow deferred splitting of arbitrary large anon folios

[Ryan Roberts]

Somehow I managed to reply only to the linux-arm-kernel list on first attempt so
resending:

On 07/07/2023 09:21, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
> 
>> With the introduction of large folios for anonymous memory, we would
>> like to be able to split them when they have unmapped subpages, in order
>> to free those unused pages under memory pressure. So remove the
>> artificial requirement that the large folio needed to be at least
>> PMD-sized.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>> Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>> ---
>>  mm/rmap.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 82ef5ba363d1..bbcb2308a1c5 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1474,7 +1474,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>>  		 * page of the folio is unmapped and at least one page
>>  		 * is still mapped.
>>  		 */
>> -		if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
>> +		if (folio_test_large(folio) && folio_test_anon(folio))
>>  			if (!compound || nr < nr_pmdmapped)
>>  				deferred_split_folio(folio);
>>  	}
> 
> One possible issue is that even for large folios mapped only in one
> process, in zap_pte_range(), we will always call deferred_split_folio()
> unnecessarily before freeing a large folio.

Hi Huang, thanks for reviewing!

I have a patch that solves this problem by determining a range of ptes covered
by a single folio and doing a "batch zap". This prevents the need to add the
folio to the deferred split queue, only to remove it again shortly afterwards.
This reduces lock contention and I can measure a performance improvement for the
kernel compilation benchmark. See [1].

However, I decided to remove it from this patch set on Yu Zhao's advice. We are
aiming for the minimal patch set to start with and wanted to focus people on
that. I intend to submit it separately later on.

[1] https://lore.kernel.org/linux-mm/20230626171430.3167004-8-ryan.roberts@xxxxxxx/

Thanks,
Ryan

> 
> Best Regards,
> Huang, Ying
> 
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