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[174.95.95.253]) by smtp.gmail.com with ESMTPSA id r2sm51753qti.4.2021.03.04.08.31.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Mar 2021 08:31:47 -0800 (PST) From: Peter Xu To: linux-man@vger.kernel.org Cc: Alejandro Colomar , Nadav Amit , Andrea Arcangeli , linux-kernel@vger.kernel.org, Michael Kerrisk , Mike Rapoport , Axel Rasmussen , Andrew Morton , Linux MM Mailing List , peterx@redhat.com Subject: [PATCH v2 2/4] userfaultfd.2: Add write-protect mode Date: Thu, 4 Mar 2021 11:31:38 -0500 Message-Id: <20210304163140.543171-3-peterx@redhat.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20210304163140.543171-1-peterx@redhat.com> References: <20210304163140.543171-1-peterx@redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="US-ASCII" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 28C57C0007E6 X-Stat-Signature: 3t68z5bh9zum17gwnh93mn65tscdutwg Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf03; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614875519-405835 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: Write-protect mode is supported starting from Linux 5.7. Signed-off-by: Peter Xu --- man2/userfaultfd.2 | 98 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 96 insertions(+), 2 deletions(-) diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 index 0cd426a8a..426307bcf 100644 --- a/man2/userfaultfd.2 +++ b/man2/userfaultfd.2 @@ -78,6 +78,30 @@ all memory ranges that were registered with the object= are unregistered and unread events are flushed. .\" .PP +Currently, userfaultfd supports two modes of registration: +.TP +.B UFFDIO_REGISTER_MODE_MISSING +When registered with +.B UFFDIO_REGISTER_MODE_MISSING +mode, the userspace will receive a page fault message when a missing pag= e is +accessed. The faulted thread will be stopped from execution until the p= age +fault is resolved from the userspace by either an +.B UFFDIO_COPY +or an +.B UFFDIO_ZEROPAGE +ioctl. +.TP +.B UFFDIO_REGISTER_MODE_WP +When registered with +.B UFFDIO_REGISTER_MODE_WP +mode, the userspace will receive a page fault message when a write-prote= cted +page is written. The faulted thread will be stopped from execution unti= l the +userspace un-write-protect the page using an +.B UFFDIO_WRITEPROTECT +ioctl. +.PP +Multiple modes can be enabled at the same time for the same memory range= . +.PP Since Linux 4.14, userfaultfd page fault message can selectively embed f= aulting thread ID information into the fault message. One needs to enable this = feature explicitly using the @@ -144,6 +168,16 @@ single threaded non-cooperative userfaultfd manager = implementations. .\" and limitations remaining in 4.11 .\" Maybe it's worth adding a dedicated sub-section... .\" +.PP +Starting from Linux 5.7, userfaultfd is able to do synchronous page dirt= y +tracking using the new write-protection register mode. One should check +against the feature bit +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP +before using this feature. Similar to the original userfaultfd missing = mode, +the write-protect mode will generate an userfaultfd message when the pro= tected +page is written. The user needs to resolve the page fault by unprotecti= ng the +faulted page and kick the faulted thread to continue. For more informat= ion, +please read the "Userfaultfd write-protect mode" section below. .SS Userfaultfd operation After the userfaultfd object is created with .BR userfaultfd (), @@ -219,6 +253,62 @@ userfaultfd can be used only with anonymous private = memory mappings. Since Linux 4.11, userfaultfd can be also used with hugetlbfs and shared memory mappings. .\" +.SS Userfaultfd write-protect mode +Since Linux 5.7, userfaultfd supports write-protect mode. The user need= s to +first check availability of this feature using +.B UFFDIO_API +ioctl against the feature bit +.BR UFFD_FEATURE_PAGEFAULT_FLAG_WP . +.PP +To register with userfaultfd write-protect mode, the user needs to initi= ate the +.B UFFDIO_REGISTER +ioctl with mode +.B UFFDIO_REGISTER_MODE_WP +set. Note that it's legal to monitor the same memory range with multipl= e +modes. For example, the user can do +.B UFFDIO_REGISTER +with the mode set to +.BR UFFDIO_REGISTER_MODE_MISSING\ |\ UFFDIO_REGISTER_MODE_WP . +When there is only +.B UFFDIO_REGISTER_MODE_WP +registered, the userspace will +.I not +receive any message when a missing page is written. Instead, the usersp= ace +will only receive a write-protect page fault message when an existing bu= t +write-protected page got written. +.PP +After the +.B UFFDIO_REGISTER +ioctl completed with +.B UFFDIO_REGISTER_MODE_WP +mode set, the user can write-protect any existing memory within the rang= e using +the ioctl +.B UFFDIO_WRITEPROTECT +where +.I uffdio_writeprotect.mode +should be set to +.BR UFFDIO_WRITEPROTECT_MODE_WP . +.PP +When a write-protect event happens, the userspace will receive a page fa= ult +message whose +.I uffd_msg.pagefault.flags +will be with +.B UFFD_PAGEFAULT_FLAG_WP +flag set. Note: since only writes can trigger such kind of fault, +write-protect messages will always be with +.B UFFD_PAGEFAULT_FLAG_WRITE +bit set too along with +.BR UFFD_PAGEFAULT_FLAG_WP . +.PP +To resolve a write-protection page fault, the user should initiate anoth= er +.B UFFDIO_WRITEPROTECT +ioctl whose +.I uffd_msg.pagefault.flags +should have the flag +.BR UFFDIO_WRITEPROTECT_MODE_WP +cleared upon the faulted page or range. +.PP +Currently, write-protect mode only supports private anonymous memory. .SS Reading from the userfaultfd structure Each .BR read (2) @@ -364,8 +454,12 @@ flag (see .BR ioctl_userfaultfd (2)) and this flag is set, this a write fault; otherwise it is a read fault. -.\" -.\" UFFD_PAGEFAULT_FLAG_WP is not yet supported. +.TP +.B UFFD_PAGEFAULT_FLAG_WP +If the address is in a range that was registered with the +.B UFFDIO_REGISTER_MODE_WP +flag, when this bit is set it means it's a write-protect fault. Otherwi= se it's +a page missing fault. .RE .TP .I pagefault.feat.pid --=20 2.26.2