Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default

[Dev Jain <dev.jain@arm.com>]

>> ---
>>   mm/huge_memory.c | 3 ---
>>   1 file changed, 3 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 26cedfcd74189..aa3ed7a86435b 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4110,9 +4110,6 @@ static bool thp_underused(struct folio *folio)
>>   	void *kaddr;
>>   	int i;
>>   
>> -	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
>> -		return false;
>> -
> I do agree with your usecase, but I am really worried about the amount of
> work and cpu time the THP shrinker will consume when max_ptes_none is 511
> (I dont have any numbers to back up my worry :)), and its less likely that
> we will have these completely zeroed out THPs (again no numbers to back up
> this statement). We have the huge_zero_folio as well which is installed on read.

How about just doing a memcmp() between huge_zero_folio and folio? We know
exactly how this folio looks like, in case of max_ptes_none == 511, if it
is to be eligible for shrinking.

>
>>   	for (i = 0; i < folio_nr_pages(folio); i++) {
>>   		kaddr = kmap_local_folio(folio, i * PAGE_SIZE);
>>   		if (!memchr_inv(kaddr, 0, PAGE_SIZE)) {