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();
