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

[Ryan Roberts]

On 10/07/2023 10:01, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts at arm.com> writes:
> 
>> On 10/07/2023 06:37, Huang, Ying wrote:
>>> 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.
>>
>> I appreciate Matthew's and others position about not wanting to merge a minimal
>> implementation while there are some fundamental features (e.g. compaction) it
>> doesn't play well with - I'm working to create a definitive list so these items
>> can be tracked and tackled.
> 
> Good to know this, Thanks!
> 
>> That said, I don't see this "batch zap" patch as an example of this. It's just a
>> performance enhancement that improves things even further than large anon folios
>> on their own. I'd rather concentrate on the core changes first then deal with
>> this type of thing later. Does that work for you?
> 
> IIUC, allocating large folios upon page fault depends on splitting large
> folios in page_remove_rmap() to avoid memory wastage.  Splitting large
> folios in page_remove_rmap() depends on "batch zap" to avoid performance
> regression in zap_pte_range().  So we need them to be done earlier.  Or
> I miss something?

My point was just that large anon folios improves performance significantly
overall, despite a small perf regression in zap_pte_range(). That regression is
reduced further by a patch from Yin Fengwei to reduce the lock contention [1].
So it doesn't seem urgent to me to get the "batch zap" change in.

I'll add it to my list, then prioritize it against the other stuff.

[1] https://lore.kernel.org/linux-mm/20230429082759.1600796-1-fengwei.yin@intel.com/

> 
> Best Regards,
> Huang, Ying