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

[David Hildenbrand <david@redhat.com>]

On 20.01.22 20:55, David Hildenbrand wrote:
> On 20.01.22 19:11, Nadav Amit wrote:
>>
>>
>>> On Jan 20, 2022, at 10:00 AM, David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 20.01.22 18:48, Nadav Amit wrote:
>>>>
>>>>> On Jan 20, 2022, at 6:15 AM, David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 17.01.22 14:31, zhangliang (AG) wrote:
>>>>>> Sure, I will do that :)
>>>>>
>>>>> I'm polishing up / testing the patches and might send something out for discussion shortly.
>>>>> Just a note that on my branch was a version with a wrong condition that should have been fixed now.
>>>>>
>>>>
>>>> Sorry for being late for the discussion.
>>>>
>>>> David, does any of it regards the lru_cache_add() reference issue that I
>>>> mentioned? [1]
>>>
>>> No, unfortunately not in that part of my work. *Maybe* we could also try
>>> to handle that reference similarly to the swapcache, but the question is
>>> if we can't wait for PageAnonExclusive.
>>>
>>> Right now I have the following in mind to get most parts working as
>>> exptected:
>>>
>>> 1. Optimize reuse logic for the swapcache as it seems to be easy
>>> 2. Streamline COW logic and remove reuse_swap_page() -- fix the CVE for
>>>   THP.
>>> 3. Introduce PageAnonExclusive and allow FOLL_PIN only on
>>>   PageAnonExclusive pages.
>>> 4. Convert O_DIRECT to FOLL_PIN
>>>
>>> We will never ever have to copy a page PageAnonExclusive page in the COW
>>> handler and can immediately reuse it without even locking the page. The
>>> existing reuse logic is essentially then used to reset PageAnonExclusive
>>> on a page (thus it makes sense to work on it) where the flag is not set
>>> anymore -- or on a fresh page if we have to copy.
>>>
>>> That implies that all these additional references won't care if your app
>>> doesn't fork() or KSM isn't active. Consequently, anything that
>>> read-protects anonymous pages will work as expected and should be as
>>> fast as it gets.
>>>
>>> Sounds good? At least to me. If only swap/migration entries wouldn't be
>>> harder to handle than I'd wish, that's why it's taking a little and will
>>> take a little longer.
>>
>> Thanks for the quick response. I would have to see the logic to set/clear
>> PageAnonExclusive to fully understand how things are handled.
>>
>> BTW, I just saw this patch form PeterZ [1] that seems to be related, as
>> it deals with changing protection on pinned pages.
> 
> Hi Nadav,
> 
> I'm trying to see how effective the following patch is with your forceswap.c [1] reproducer.
> 
> commit b08d494deb319a63b7c776636b960258c48775e1
> Author: David Hildenbrand <david@redhat.com>
> Date:   Fri Jan 14 09:29:52 2022 +0100
> 
>     mm: optimize do_wp_page() for exclusive pages in the swapcache
>     
>     Let's optimize for a page with a single user that has been added to the
>     swapcache. Try removing the swapcache reference if there is hope that
>     we're the exclusive user, but keep the page_count(page) == 1 check in
>     place.
>     
>     Avoid using reuse_swap_page(), we'll streamline all reuse_swap_page()
>     users next.
>     
>     While at it, remove the superfluous page_mapcount() check: it's
>     implicitly covered by the page_count() for ordinary anon pages.
>     
>     Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index f306e698a1e3..d9186981662a 100644
> --- a/mm/memory.c
> +++ 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 should stop working for today, I messed up the counter names *cries in
German* :(

anon_wp_reuse 1799
anon_wp_copy_count 1
anon_wp_copy_count_early 983396
anon_wp_copy_lock 0

which makes *a lot* more sense and might indicate the PageLRU() issue.

-- 
Thanks,

David / dhildenb