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

[David Hildenbrand]

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 :)

-- 
Thanks,

David / dhildenb