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[2003:cb:c706:d200:2d99:b40f:ff2c:cf51]) by smtp.gmail.com with ESMTPSA id az25-20020a05600c601900b0037bdd0430c9sm7894819wmb.16.2022.02.21.07.31.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Feb 2022 07:31:35 -0800 (PST) Message-ID: <4bb1d132-5cf4-bd8c-b0b9-c2c8e0678308@redhat.com> Date: Mon, 21 Feb 2022 16:31:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH v2] userfaultfd: provide unmasked address on page-fault To: Nadav Amit , Andrew Morton Cc: linux-mm@kvack.org, Nadav Amit , Andrea Arcangeli , Mike Rapoport , Peter Xu , Jan Kara References: <20220218041003.3508-1-namit@vmware.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220218041003.3508-1-namit@vmware.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-Server: rspam01 X-Rspamd-Queue-Id: B38DF140003 X-Rspam-User: Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=D7PMnnmH; spf=none (imf26.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: ejmxbf3oux6jdwd7uue8qfpmhkqjk44p X-HE-Tag: 1645457499-182604 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 18.02.22 05:10, Nadav Amit wrote: > From: Nadav Amit > > Userfaultfd is supposed to provide the full address (i.e., unmasked) of > the faulting access back to userspace. However, that is not the case for > quite some time. > > Even running "userfaultfd_demo" from the userfaultfd man page provides > the wrong output (and contradicts the man page). Notice that > "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000) > and not the first read address (0x7fc5e30b300f). > > Address returned by mmap() = 0x7fc5e30b3000 > > fault_handler_thread(): > poll() returns: nready = 1; POLLIN = 1; POLLERR = 0 > UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000 > (uffdio_copy.copy returned 4096) > Read address 0x7fc5e30b300f in main(): A > Read address 0x7fc5e30b340f in main(): A > Read address 0x7fc5e30b380f in main(): A > Read address 0x7fc5e30b3c0f in main(): A > > The exact address is useful for various reasons and specifically for > prefetching decisions. If it is known that the memory is populated by > certain objects whose size is not page-aligned, then based on the > faulting address, the uffd-monitor can decide whether to prefetch and > prefault the adjacent page. > > This bug has been for quite some time in the kernel: since commit > 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address") > vmf->virtual_address"), which dates back to 2016. A concern has been > raised that existing userspace application might rely on the old/wrong > behavior in which the address is masked. Therefore, it was suggested to > provide the masked address unless the user explicitly asks for the exact > address. > > Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct > userfaultfd to provide the exact address. Add a new "real_address" field > to vmf to hold the unmasked address. Provide the address to userspace > accordingly. > > Cc: David Hildenbrand > Cc: Andrea Arcangeli > Cc: Mike Rapoport > Cc: Peter Xu > Cc: Jan Kara > Signed-off-by: Nadav Amit Thanks Nadav! Reviewed-by: David Hildenbrand -- Thanks, David / dhildenb