Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jan 20, 2022 at 08:55:12PM +0100, David Hildenbrand wrote:
> >>> David, does any of it regards the lru_cache_add() reference issue that I
> >>> mentioned? [1]

> +++ b/mm/memory.c
> @@ -3291,19 +3291,28 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>         if (PageAnon(vmf->page)) {
>                 struct page *page = vmf->page;
>  
> -               /* PageKsm() doesn't necessarily raise the page refcount */
> -               if (PageKsm(page) || page_count(page) != 1)
> +               /*
> +                * PageKsm() doesn't necessarily raise the page refcount.
> +                *
> +                * These checks are racy as long as we haven't locked the page;
> +                * they are a pure optimization to avoid trying to lock the page
> +                * and trying to free the swap cache when there is little hope
> +                * it will actually result in a refcount of 1.
> +                */
> +               if (PageKsm(page) || page_count(page) > 1 + PageSwapCache(page))
>                         goto copy;
>                 if (!trylock_page(page))
>                         goto copy;
> -               if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
> +               if (PageSwapCache(page))
> +                       try_to_free_swap(page);
> +               if (PageKsm(page) || page_count(page) != 1) {
>                         unlock_page(page);
>                         goto copy;
>                 }
>                 /*
> -                * Ok, we've got the only map reference, and the only
> -                * page count reference, and the page is locked,
> -                * it's dark out, and we're wearing sunglasses. Hit it.
> +                * Ok, we've got the only page reference from our mapping
> +                * and the page is locked, it's dark out, and we're wearing
> +                * sunglasses. Hit it.
>                  */
>                 unlock_page(page);
>                 wp_page_reuse(vmf);
> 
> 
> I added some vmstats that monitor various paths. After one run of
> 	./forceswap 2 1000000 1
> I'm left with a rough delta (including some noise) of
> 	anon_wp_copy_count 1799
> 	anon_wp_copy_count_early 1
> 	anon_wp_copy_lock 983396
> 	anon_wp_reuse 0
> 
> The relevant part of your reproducer is
> 
> 	for (i = 0; i < nops; i++) {
> 		if (madvise((void *)p, PAGE_SIZE * npages, MADV_PAGEOUT)) {
> 			perror("madvise");
> 			exit(-1);
> 		}
> 
> 		for (j = 0; j < npages; j++) {
> 			c = p[j * PAGE_SIZE];
> 			c++;
> 			time -= rdtscp();
> 			p[j * PAGE_SIZE] = c;
> 			time += rdtscp();
> 		}
> 	}
> 
> For this specific reproducer at least, the page lock seems to be the thingy that prohibits
> reuse if I interpret the numbers correctly. We pass the initial page_count() check.
> 
> Haven't looked into the details, and I would be curious how that performs with actual
> workloads, if we can reproduce similar behavior.

I don't see how that patch addresses the lru issue.  Wouldn't we need
something like ...

	if (!PageLRU(page))
		lru_add_drain_all();