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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id g22-20020a37e216000000b007677f66b160sm581885qki.124.2023.07.20.13.07.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 20 Jul 2023 13:07:57 -0700 (PDT) Date: Thu, 20 Jul 2023 16:07:56 -0400 From: Peter Xu To: Hillf Danton Cc: Dimitris Siakavaras , Axel Rasmussen , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released Message-ID: References: <79375b71-db2e-3e66-346b-254c90d915e2@cslab.ece.ntua.gr> <20230719211631.890995-1-axelrasmussen@google.com> <20230720103534.312-1-hdanton@sina.com> MIME-Version: 1.0 In-Reply-To: <20230720103534.312-1-hdanton@sina.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: EB641A001E X-Rspam-User: X-Stat-Signature: 4usxpefoups3u1u8wagxdiifh6sc5kbx X-Rspamd-Server: rspam03 X-HE-Tag: 1689883680-432932 X-HE-Meta: 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 lcIRpIB1 /drSsRc44rQTrHwjTsnHxJHypiIRpvRyXbkNhEMwOImdILJwSey2+gtBjCBekk1uWu7gmBeYL8UGbE3k9IjBxc1Qvz2uJbA9RnTPhoWrWyuYbK9zhqqaqdPWSSgVyYAz1+YXPmIJGaq7sfP6KvOksoKPltagO7oIEIMSf2AXjFXdcvTdHVUT/mUTW6J5Z11kzEr/052E/zmg2XqJEtd3mVo+ILNIbdXgrnszh7+Gfu8VZ3FCgAOaFUZkmZbUqHItsxlnepXjfwyTqNnzKk019OdeuarglovXNlluBsWxsGifiXX1uOCioCcwpFnObhWey7azlOZDTmFMG8Mu0VD38ZnNZvkEq8EPrBtOoXgdJ5kJDVQDQ+oRT6G5rjdWnJhShBjvvw29OI6rjCY7Z18+uhESwaa4h3j4fK7NVhK3KuGfNgHC1OFbs9LXpXA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.055732, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jul 20, 2023 at 06:35:34PM +0800, Hillf Danton wrote: > On Wed, Jul 19, 2023 at 2:16 PM Axel Rasmussen wrote: > > > > I think perhaps the right thing to do is to have handle_userfault() release > > mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that > > appropriately? But, some investigation is required to be sure that's okay to do > > in the other non-GUP ways we can end up in handle_userfault(). > > See if making kworker special works. > > --- x/fs/userfaultfd.c > +++ y/fs/userfaultfd.c > @@ -457,6 +457,8 @@ vm_fault_t handle_userfault(struct vm_fa > * close the uffd. > */ > ret = VM_FAULT_NOPAGE; > + if (current->flags & PF_WQ_WORKER) > + ret = VM_FAULT_OOM; > goto out; > } Sorry this won't work - we need userfault to work with all forms of kworkers, especially including kvm async pf. Thanks. -- Peter Xu