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=-14.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 326CAC433DB for ; Tue, 23 Mar 2021 18:20:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D3E67619BB for ; Tue, 23 Mar 2021 18:20:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231420AbhCWSTc (ORCPT ); Tue, 23 Mar 2021 14:19:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42676 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231243AbhCWSTS (ORCPT ); Tue, 23 Mar 2021 14:19:18 -0400 Received: from mail-wr1-x432.google.com (mail-wr1-x432.google.com [IPv6:2a00:1450:4864:20::432]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 462B0C061574; Tue, 23 Mar 2021 11:19:15 -0700 (PDT) Received: by mail-wr1-x432.google.com with SMTP id x16so21777840wrn.4; Tue, 23 Mar 2021 11:19:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=ca1Tuwhu5omAYPstfde+it7eK74ixpm8NMvNQMQFKWE=; b=SoQHhDCi88HemNMTFU8Xmq/0h+uYUn7HXkpJFAEqrZbk52+0U+n7mTSC08d2oZV0KS iWfdH7UItmXdeEyfaSdor4Ws1SvIBQst2GliHMtdVoBp1vixbD61zKveaadRdbhoLJw7 7upSHgGEp+cdEhfuR5NEq9oybIn8j5v3k4ao7vloe6UpPY5zThYFfyBzYQJsOHgnQUh2 wxb1w61FmThHDVySdE6KFafleUVHFD4MR8npKFMVvCWAa8mSdqoVzhXl9CnN9sj52qw/ mb7dtDI0t0gHRinnd2MMgZk89tnkFLK3dVIKxel6FXmv5lm20ieTPwvuoBreGMxNILOp Br/A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=ca1Tuwhu5omAYPstfde+it7eK74ixpm8NMvNQMQFKWE=; b=IuZtwRNKxkQPgO/QiLFxvVOusDbaDN7DivrhpVj2jUaiIWFbgy2L+UpdhFA9AILOtR 6KfoApFNeU7BIjXDvVhTcNzF6OqwES51IIZY2dcwSP6yUvQ122uIzAsZsIlmJx4xTAiI 7WXX+Gw98kq1ult9p5mOXYSAWHlsYwcqLusowmyATXP7RiZuaSz1tWDf8IF6F0wTUQUN tq5jeLhIvIeHImjUy0SdkyoMWsh7QAxkM/GES9Qs0xuw//sHw/VnQ5fh4ueC4SZYmQgu Tdmp6m2mhkjbi1/vUmdKrHcwZfV749ReBrCXXCm6xtu3fjlw5ex9j7kPz7MyveojDM5I zNpA== X-Gm-Message-State: AOAM533yKLYQKUaYuKu2Istq/527fUxnE/tDG2X1saFc+50qGs8tSGC5 mAKVJ4+tZaejwFqB0brLDch5bUMGbgE= X-Google-Smtp-Source: ABdhPJzhYMIoiz7bly4TNvwy50fv9d5a96W9TKxOnzC0CLnSERhxeq1TA/+QghHlrHPr0JbBUOOzng== X-Received: by 2002:a05:6000:18f:: with SMTP id p15mr5350393wrx.23.1616523554001; Tue, 23 Mar 2021 11:19:14 -0700 (PDT) Received: from [10.8.0.206] ([195.53.121.100]) by smtp.gmail.com with ESMTPSA id c9sm24034820wrr.78.2021.03.23.11.19.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Mar 2021 11:19:13 -0700 (PDT) Subject: Re: [PATCH v4 2/4] userfaultfd.2: Add write-protect mode To: Peter Xu , linux-man@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport Cc: Andrea Arcangeli , Axel Rasmussen , Michael Kerrisk , Nadav Amit , Andrew Morton References: <20210322220848.52162-1-peterx@redhat.com> <20210322220848.52162-3-peterx@redhat.com> From: "Alejandro Colomar (man-pages)" Message-ID: <9c5a6fd4-e0f6-026a-e6a2-db34234a56c8@gmail.com> Date: Tue, 23 Mar 2021 19:19:12 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210322220848.52162-3-peterx@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, Please see a few more comments below. Thanks, Alex On 3/22/21 11:08 PM, Peter Xu wrote: > Write-protect mode is supported starting from Linux 5.7. > > Signed-off-by: Peter Xu > --- > man2/userfaultfd.2 | 104 ++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 102 insertions(+), 2 deletions(-) > > diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 > index 555e37409..8ad4a71b5 100644 > --- a/man2/userfaultfd.2 > +++ b/man2/userfaultfd.2 > @@ -78,6 +78,32 @@ all memory ranges that were registered with the object are unregistered > and unread events are flushed. > .\" > .PP > +Userfaultfd supports two modes of registration: > +.TP > +.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)" > +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 > +.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)" > +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 Here you use un-write-protect, but in the other patch you use write-unprotect. Please, use a consistent wording if it's the same thing (if there are other similar things with different wordings in different pages, please fix them too, but I didn't see more of those). If there's already a wording for that in any page, please reuse it (I ignore it). > +.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 +170,17 @@ 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 The previous paragraph uses "Siince Linux 4.14". For consistency, please use that same wording here. > +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 refer to "Userfaultfd write-protect mode" section. > .SS Userfaultfd operation > After the userfaultfd object is created with > .BR userfaultfd (), > @@ -219,6 +256,65 @@ 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 5.7) > +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 > +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP > +before using this feature. > +.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 bit > +.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 > +.B UFFDIO_WRITEPROTECT_MODE_WP > +cleared upon the faulted page or range. > +.PP > +Write-protect mode only supports private anonymous memory. > .SS Reading from the userfaultfd structure > Each > .BR read (2) > @@ -364,8 +460,12 @@ 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 > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es/