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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id o1-20020a63f141000000b00478eb777d18sm19268912pgk.72.2023.01.03.16.57.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Jan 2023 16:57:13 -0800 (PST) Date: Wed, 4 Jan 2023 00:57:09 +0000 From: Sean Christopherson To: James Houghton Cc: David Matlack , Peter Xu , Andrea Arcangeli , Paolo Bonzini , Axel Rasmussen , Linux MM , kvm , chao.p.peng@linux.intel.com, Oliver Upton Subject: Re: [RFC] Improving userfaultfd scalability for live migration Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C926C100009 X-Stat-Signature: tzrnhoqbdeczrczjg846a5f4pjdepumq X-HE-Tag: 1672793834-483017 X-HE-Meta: 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 iP00gJ93 9NANn3TGvwOxnbLEyMnJBkXzajqNl7NHL0K35NChLICiZzwiz/jmxdMw2u2TLOV+r/ah/5JynzAMKrMARU2jGWEBvDJAgc9ER8qtH X-Bogosity: Ham, tests=bogofilter, spamicity=0.087305, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Dec 08, 2022, James Houghton wrote: > - For the no-slow-GUP choice, if someone MADV_DONTNEEDed memory and we > didn't know about it, we would get stuck in MADV_POPULATE_WRITE. By > using UFFD_FEATURE_THREAD_ID, we can tell if we got a userfault for a > thread that is in the middle of a MADV_POPULATE_WRITE, and we can try > to unblock the thread by doing an extra UFFDIO_CONTINUE. > > - For the PF_NO_UFFD_WAIT choice, if someone MADV_DONTNEEDed memory, > we would just keep trying to start the vCPU without doing anything (we > assume some other thread has UFFDIO_CONTINUEd for us). This is > basically the same as if we were stuck in MADV_POPULATE_WRITE, and we > can try to unblock the thread in a fashion similar to how we would in > the other case. > > So really these approaches have similar requirements for what > userspace needs to track. So I think I prefer the no-slow-GUP approach > then. Are you planning on sending a patch (RFC?) for the no-slow-GUP approach? It sounds like there's a rough consensus that that's a viable, minimally invasive solution.