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

[Nadav Amit <nadav.amit@gmail.com>]

> On Jan 20, 2022, at 12:37 PM, David Hildenbrand <david@redhat.com> wrote:
> 
> On 20.01.22 21:09, David Hildenbrand wrote:
>> On 20.01.22 21:07, Matthew Wilcox wrote:
>>> 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();
>>> 
> 
> lru_add_drain_all() takes a mutex ... best we can do I guess is drain
> the local CPU using lru_add_drain(). I'll go play with it and see what
> breaks :)
> 

I did hack something similar and it solved the problem, but I felt it is
a hack. If the thread is scheduled on another core, or if the write fault
is triggered by another thread it wouldn’t work.

If you look for a real-world workload that behaves similarly, you can try
memcached with memory pressure and low latency device (I used 
pmem-emulated). This is the workload in which I encountered the issue
first.