linux-kernel - Re: [PATCH] mm: reuse the unshared swapcache page in do_wp

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Date:   Thu, 20 Jan 2022 21:09:46 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        "zhangliang (AG)" <zhangliang5@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        wangzhigang17@...wei.com,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

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();
> 

See my other reply "No, unfortunately not in that part of my work.".

Would the lru handling help here where we force swapout of a single
page, reuse code passes the "page_count(page) > 1 + PageSwapCache(page)"
check but fails to lock the page?

-- 
Thanks,

David / dhildenb