From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0ED23C4332B for ; Thu, 4 Mar 2021 16:34:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E934D64F47 for ; Thu, 4 Mar 2021 16:34:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237975AbhCDQda (ORCPT ); Thu, 4 Mar 2021 11:33:30 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:48311 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237838AbhCDQdQ (ORCPT ); Thu, 4 Mar 2021 11:33:16 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614875511; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ZyOpGEITNqQDEfKjmjcv27JyIGMyq+vfMvCfDr9TWdA=; b=brMkAzOkUZwROih22Nrf7QiNpErzMnYeeDzu2SGfqvUzXBHVRCjrSSmcAVo5MbrgLGhJH7 gCS23Jxesa4bOHOr+7T4sPfkMbJTC/45N2QXSrf/qeIn+ZPOgZIEARd5gt7SIoagkF3yXf URwSSpwxM7+aenGh3TNLDDetJWcNrkA= Received: from mail-qk1-f199.google.com (mail-qk1-f199.google.com [209.85.222.199]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-237-FUQCvVr-OGijuRMKGL01kA-1; Thu, 04 Mar 2021 11:31:49 -0500 X-MC-Unique: FUQCvVr-OGijuRMKGL01kA-1 Received: by mail-qk1-f199.google.com with SMTP id y79so23522452qka.23 for ; Thu, 04 Mar 2021 08:31:48 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=ZyOpGEITNqQDEfKjmjcv27JyIGMyq+vfMvCfDr9TWdA=; b=R0LRzBaNFUn2gnkPLF2Mys7zc834rwK8s2W5h/1yL/yFbJn9TZ/9JAizJDy4CqRaDH lNhnNtR/GIAJAM0GMNwclylqbaKDQ3gz5NLEUEwHj9KtYVSw/ExNcq9+jU5jsFFRXumZ evDutJ/HowsML7d4MXtmmToekDjIb6nV/tqtZL6ztvBn4xPSGGQUK5RMdvLH4pyGpPDe uItNjCpXRmQzTK0Jghf2giyFmMhnUVjlLkE6iMAl+pLZMRPqPuEN0rY0pCmFix1ckI1q jJwTnIgfBkG2YvxzMIaibTiaceFjyDwLA1WHcU09PefRudvzUaOzFv02keLFdCHJNNR5 PcMA== X-Gm-Message-State: AOAM53344Mp9vb4kjdT/e4PsyXeRVJiXFANezxECfIalW/v52GihHqAC KAv83xuGzFGVxbogYTD2sF39QJeemUrhotHICdV8E76SINEm6elnPH/4Na2zvCj5BLFJtbcTDwr +MCV7dM1YUh9Qw4KgTFGT9bY+ X-Received: by 2002:ac8:6f53:: with SMTP id n19mr4705850qtv.179.1614875508059; Thu, 04 Mar 2021 08:31:48 -0800 (PST) X-Google-Smtp-Source: ABdhPJxzgwyOHBLorA9J+eec8/2ydUNPZ5sjcWOkeq5wdSnZvKOKIVu115dffGwg3GqEHPXi9C5SdA== X-Received: by 2002:ac8:6f53:: with SMTP id n19mr4705816qtv.179.1614875507692; Thu, 04 Mar 2021 08:31:47 -0800 (PST) Received: from xz-x1.redhat.com (bras-vprn-toroon474qw-lp130-25-174-95-95-253.dsl.bell.ca. [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 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 page is +accessed. The faulted thread will be stopped from execution until the page +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-protected +page is written. The faulted thread will be stopped from execution until 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 faulting 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 dirty +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 protected +page is written. The user needs to resolve the page fault by unprotecting the +faulted page and kick the faulted thread to continue. For more information, +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 needs 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 initiate 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 multiple +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 userspace +will only receive a write-protect page fault message when an existing but +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 range 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 fault +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 another +.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. Otherwise it's +a page missing fault. .RE .TP .I pagefault.feat.pid -- 2.26.2