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[91.12.103.110]) by smtp.gmail.com with ESMTPSA id r18sm1744195wrs.47.2021.10.08.01.05.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 08 Oct 2021 01:05:37 -0700 (PDT) To: Nadav Amit , Andrew Morton , Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Nadav Amit , Andrea Arcangeli , Mike Rapoport , Jan Kara , stable@vger.kernel.org References: <20211007235055.469587-1-namit@vmware.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH] mm/userfaultfd: provide unmasked address on page-fault Message-ID: Date: Fri, 8 Oct 2021 10:05:36 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20211007235055.469587-1-namit@vmware.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B9BB2103C5DA X-Stat-Signature: 9e5ppm9bcgwt8jto9oe8etqshc9iprtb Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=BALTjwHY; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf13.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-HE-Tag: 1633680340-626491 Content-Transfer-Encoding: quoted-printable 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 08.10.21 01:50, Nadav Amit wrote: > From: Nadav Amit >=20 > Userfaultfd is supposed to provide the full address (i.e., unmasked) of > the faulting access back to userspace. However, that is not the case fo= r > quite some time. >=20 > 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. >=20 > Address returned by mmap() =3D 0x7fc5e30b3000 >=20 > fault_handler_thread(): > poll() returns: nready =3D 1; POLLIN =3D 1; POLLERR =3D 0 > UFFD_EVENT_PAGEFAULT event: flags =3D 0; address =3D 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 >=20 > Add a new "real_address" field to vmf to hold the unmasked address. It > is possible to keep the unmasked address in the existing address field > (and mask whenever necessary) instead, but this is likely to cause > backporting problems of this patch. Can we be sure that no existing users will rely on this behavior that=20 has been the case since end of 2016 IIRC, one year after UFFD was=20 upstreamed? I do wonder what the official ABI nowadays is, because man=20 pages aren't necessarily the source of truth. I checked QEMU (postcopy live migration), and I think it should be fine=20 with this change. If we don't want to change the current ABI behavior, we could add a new=20 feature flag to change behavior. @Peter, what are your thoughts? --=20 Thanks, David / dhildenb