Re: [PATCH] mm: free idle swap cache page after COW

[Matthew Wilcox]

On Tue, Jun 01, 2021 at 01:31:43PM +0800, Huang Ying wrote:
> With commit 09854ba94c6a ("mm: do_wp_page() simplification"), after
> COW, the idle swap cache page (neither the page nor the corresponding
> swap entry is mapped by any process) will be left in the LRU list,
> even if it's in the active list or the head of the inactive list.  So,
> the page reclaimer may take quite some overhead to reclaim these
> actually unused pages.
> 
> To help the page reclaiming, in this patch, after COW, the idle swap
> cache page will be tried to be freed.  To avoid to introduce much
> overhead to the hot COW code path,
> 
> a) there's almost zero overhead for non-swap case via checking
>    PageSwapCache() firstly.
> 
> b) the page lock is acquired via trylock only.
> 
> To test the patch, we used pmbench memory accessing benchmark with
> working-set larger than available memory on a 2-socket Intel server
> with a NVMe SSD as swap device.  Test results shows that the pmbench
> score increases up to 23.8% with the decreased size of swap cache and
> swapin throughput.

So 2 percentage points better than my original idea?  Sweet.

> diff --git a/mm/memory.c b/mm/memory.c
> index 2b7ffcbca175..d44425820240 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3104,6 +3104,8 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  				munlock_vma_page(old_page);
>  			unlock_page(old_page);
>  		}
> +		if (page_copied)
> +			free_swap_cache(old_page);
>  		put_page(old_page);
>  	}
>  	return page_copied ? VM_FAULT_WRITE : 0;

Why not ...

		if (page_copied)
			free_page_and_swap_cache(old_page);
		else
			put_page(old_page);

then you don't need to expose free_swap_cache().  Or does the test for
huge_zero_page mess this up?