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

[Matthew Wilcox]

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:

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

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