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

[Huang, Ying]

Ryan Roberts <ryan.roberts at arm.com> writes:

> 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 at arm.com> 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 at arm.com>
>>> Reviewed-by: Yu Zhao <yuzhao at google.com>
>>> Reviewed-by: Yin Fengwei <fengwei.yin at intel.com>
>>> ---
>>>  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@arm.com/

Thanks for your information!  "batch zap" can solve the problem.

And, I agree with Matthew's comments to fix the large folios interaction
issues before merging the patches to allocate large folios as in the
following email.

https://lore.kernel.org/linux-mm/ZKVdUDuwNWDUCWc5@casper.infradead.org/

If so, we don't need to introduce the above problem or a large patchset.

Best Regards,
Huang, Ying