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[2003:cb:c70e:5800:eeb:dae2:b1c0:f5d1]) by smtp.gmail.com with ESMTPSA id bg26sm8586342wmb.48.2022.01.20.12.18.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Jan 2022 12:18:33 -0800 (PST) Message-ID: <0c48872e-f927-d36a-b9b6-10e2e2cacb3b@redhat.com> Date: Thu, 20 Jan 2022 21:18:32 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page From: David Hildenbrand To: Nadav Amit Cc: "zhangliang (AG)" , Andrew Morton , Linux-MM , Linux Kernel Mailing List , wangzhigang17@huawei.com, Matthew Wilcox , Linus Torvalds References: <20220113140318.11117-1-zhangliang5@huawei.com> <172ccfbb-7e24-db21-7d84-8c8d8c3805fd@redhat.com> <9cd7eee2-91fd-ddb8-e47d-e8585e5baa05@redhat.com> <747ff31c-6c9e-df6c-f14d-c43aa1c77b4a@redhat.com> <8931808d-db61-0f06-ceb3-f48a83b1f74c@redhat.com> <6225EAFF-B323-4DC5-AC4C-885B29ED7261@gmail.com> <9071d5a8-ed2d-5cf5-5526-43fe7dd377ec@redhat.com> Organization: Red Hat In-Reply-To: <9071d5a8-ed2d-5cf5-5526-43fe7dd377ec@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 53A8E140019 X-Stat-Signature: dhupcdhfsa9dfp4ec3fy4418g1bh8aym Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="i/j50nJx"; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf23.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam06 X-HE-Tag: 1642709917-759437 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 wrote: >>> >>> On 20.01.22 18:48, Nadav Amit wrote: >>>> >>>>> On Jan 20, 2022, at 6:15 AM, David Hildenbrand 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 > 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 > > 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