Re: [PATCH RFC v2 2/9] mm: optimize do_wp_page() for fresh pages in local LRU pagevecs

[David Hildenbrand <david@redhat.com>]

On 26.01.22 15:31, Matthew Wilcox wrote:
> On Wed, Jan 26, 2022 at 10:55:50AM +0100, David Hildenbrand wrote:
>> diff --git a/mm/memory.c b/mm/memory.c
>> index bcd3b7c50891..61d67ceef734 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3298,7 +3298,17 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>>  		 *
>>  		 * PageKsm() doesn't necessarily raise the page refcount.
>>  		 */
>> -		if (PageKsm(page) || page_count(page) > 1 + PageSwapCache(page))
>> +		if (PageKsm(page))
>> +			goto copy;
>> +		if (page_count(page) > 1 + PageSwapCache(page) + !PageLRU(page))
>> +			goto copy;
>> +		if (!PageLRU(page))
>> +			/*
>> +			 * Note: We cannot easily detect+handle references from
>> +			 * remote LRU pagevecs or references to PageLRU() pages.
>> +			 */
>> +			lru_add_drain();
>> +		if (page_count(page) > 1 + PageSwapCache(page))
>>  			goto copy;
> 
> I worry we're starting to get too accurate here.  How about:
> 

In that simplified case we'll essentially trylock_page() and for most
pages that are ordinarily shared by e.g., 2 processes.

> 		if (PageKsm(page) || page_count(page) > 3)
> 			goto copy;
> 		if (!PageLRU(page))
> 			lru_add_drain();
> 		if (!trylock_page(page))
> 			goto copy;
> ...
> 

I think we might at least want the

if (page_count(page) > 1 + PageSwapCache(page))
	goto copy;

check here.

>>  		if (!trylock_page(page))
>>  			goto copy;
>> -- 
>> 2.34.1
>>
> 


-- 
Thanks,

David / dhildenb