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=-23.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, 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_IN_DEF_DKIM_WL 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 A291AC432BE for ; Tue, 27 Jul 2021 16:33:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B0C161B93 for ; Tue, 27 Jul 2021 16:33:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229680AbhG0QdM (ORCPT ); Tue, 27 Jul 2021 12:33:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48834 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229441AbhG0QdK (ORCPT ); Tue, 27 Jul 2021 12:33:10 -0400 Received: from mail-io1-xd2f.google.com (mail-io1-xd2f.google.com [IPv6:2607:f8b0:4864:20::d2f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AD824C061760 for ; Tue, 27 Jul 2021 09:33:10 -0700 (PDT) Received: by mail-io1-xd2f.google.com with SMTP id h1so16652543iol.9 for ; Tue, 27 Jul 2021 09:33:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=1MsdmDXnsLB9eMjzkHg2OvMvy38XWDE8T/3fr7aHZoY=; b=B+LNu3SBhnvYwMIPPScScuyJR6XFl67VzmflzeFkoGhcl2COqjZrmhyq8Q83MVY19b XgiJmkMJt4hmKR1fsA4ePjC0sZejI0XAeNce/xyZGQYD3AR2qHobvljw6A6f+Ao3JoQS nsbVdCn0ayRp/TjBu2uaISN97hjnMwUHEL1lr+yc9J8qkUGqg5wmnlJzHpy5FYyqJeB5 Jd2XjaGNzyEU2taYgNf+f8KyAOC5ecRRNSNLrmsb3rlkO9IEGbLNE/WlE7SFJTHrukC8 7bTN0zgx7j0QQN3rP+tcrlmOY7HTIA5H+nfO/3WwOPofMTPN1Plxgxk17u3dmSuoM3rX bG7w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=1MsdmDXnsLB9eMjzkHg2OvMvy38XWDE8T/3fr7aHZoY=; b=ntPwI0DkKTpYzxAjVOJZ7ljt41pQWeHNEQq1SjND4pbPrxHCgyaUwzN/H85rQof5gX jP0Rw7T/WbCf1k3K+vmToiMNthD8WpArsHW+/TgYNvM4KnVrocmJvL721YWqKxyAqwvm sdsDd0oCxsfrjkOmwMCTFJyj/Yt1cXPk2G6/cOm7CpKoCbSn4xWdNoue+QRvlinWXeRO Dwn0t41/DkDeKmsyT51WYG8+1ndqDoD9QgQKDDIpmosaaGSdO+IgcbHFznxxYV8B+h76 KR8R7ZlCsWd/V5nKWRv6xJiHKeHYzyvWpxJO0xqjwvbUT/ivbhJs9XjKiIdJjABu3Lcj QlHg== X-Gm-Message-State: AOAM532eI0Qu+41BODEaiJU1R0Ca7jnR9ZeMj/oHP/JZIz2rF/Yfmt42 I/nL//J3J9FJ3VjClQBaseLdygwwSKZTPIVz7dkaqA== X-Google-Smtp-Source: ABdhPJxxhqzZ+lE7Z8iozhmIr1UPAu+Agxn3N21CdOpPn8l5oiwwB0MyTPSRGUcmvSR43bCSskVdPC+ub6laDk6xRiw= X-Received: by 2002:a05:6602:2245:: with SMTP id o5mr20076442ioo.20.1627403589829; Tue, 27 Jul 2021 09:33:09 -0700 (PDT) MIME-Version: 1.0 References: <20210604195622.1249588-1-axelrasmussen@google.com> In-Reply-To: <20210604195622.1249588-1-axelrasmussen@google.com> From: Axel Rasmussen Date: Tue, 27 Jul 2021 09:32:34 -0700 Message-ID: Subject: Re: [PATCH v2] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode To: Andrea Arcangeli , Andrew Morton , Hugh Dickins , Mike Kravetz , Peter Xu Cc: LKML , linux-man@vger.kernel.org, Linux MM Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Any remaining issues with this patch? I just realized today it was never merged. 5.13 (which contains this new feature) was released some weeks ago. On Fri, Jun 4, 2021 at 12:56 PM Axel Rasmussen wrote: > > Userfaultfd minor fault mode is supported starting from Linux 5.13. > > This commit adds a description of the new mode, as well as the new ioctl > used to resolve such faults. The two go hand-in-hand: one can't resolve > a minor fault without continue, and continue can't be used to resolve > any other kind of fault. > > This patch covers just the hugetlbfs implementation (in 5.13). Support > for shmem is forthcoming, but as it has not yet made it into a kernel > release candidate, it will be added in a future commit. > > Signed-off-by: Axel Rasmussen > --- > man2/ioctl_userfaultfd.2 | 125 ++++++++++++++++++++++++++++++++++++--- > man2/userfaultfd.2 | 79 ++++++++++++++++++++----- > 2 files changed, 182 insertions(+), 22 deletions(-) > > diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2 > index 504f61d4b..7b990c24a 100644 > --- a/man2/ioctl_userfaultfd.2 > +++ b/man2/ioctl_userfaultfd.2 > @@ -214,6 +214,10 @@ memory accesses to the regions registered with userfaultfd. > If this feature bit is set, > .I uffd_msg.pagefault.feat.ptid > will be set to the faulted thread ID for each page-fault message. > +.TP > +.BR UFFD_FEATURE_MINOR_HUGETLBFS " (since Linux 5.13)" > +If this feature bit is set, the kernel supports registering userfaultfd ranges > +in minor mode on hugetlbfs-backed memory areas. > .PP > The returned > .I ioctls > @@ -240,6 +244,11 @@ operation is supported. > The > .B UFFDIO_WRITEPROTECT > operation is supported. > +.TP > +.B 1 << _UFFDIO_CONTINUE > +The > +.B UFFDIO_CONTINUE > +operation is supported. > .PP > This > .BR ioctl (2) > @@ -278,14 +287,8 @@ by the current kernel version. > (Since Linux 4.3.) > Register a memory address range with the userfaultfd object. > The pages in the range must be "compatible". > -.PP > -Up to Linux kernel 4.11, > -only private anonymous ranges are compatible for registering with > -.BR UFFDIO_REGISTER . > -.PP > -Since Linux 4.11, > -hugetlbfs and shared memory ranges are also compatible with > -.BR UFFDIO_REGISTER . > +Please refer to the list of register modes below for the compatible memory > +backends for each mode. > .PP > The > .I argp > @@ -324,9 +327,16 @@ the specified range: > .TP > .B UFFDIO_REGISTER_MODE_MISSING > Track page faults on missing pages. > +Since Linux 4.3, only private anonymous ranges are compatible. > +Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible. > .TP > .B UFFDIO_REGISTER_MODE_WP > Track page faults on write-protected pages. > +Since Linux 5.7, only private anonymous ranges are compatible. > +.TP > +.B UFFDIO_REGISTER_MODE_MINOR > +Track minor page faults. > +Since Linux 5.13, only hugetlbfs ranges are compatible. > .PP > If the operation is successful, the kernel modifies the > .I ioctls > @@ -735,6 +745,105 @@ or not registered with userfaultfd write-protect mode. > .TP > .B EFAULT > Encountered a generic fault during processing. > +.\" > +.SS UFFDIO_CONTINUE > +(Since Linux 5.13.) > +Resolve a minor page fault by installing page table entries for existing pages > +in the page cache. > +.PP > +The > +.I argp > +argument is a pointer to a > +.I uffdio_continue > +structure as shown below: > +.PP > +.in +4n > +.EX > +struct uffdio_continue { > + struct uffdio_range range; /* Range to install PTEs for and continue */ > + __u64 mode; /* Flags controlling the behavior of continue */ > + __s64 mapped; /* Number of bytes mapped, or negated error */ > +}; > +.EE > +.in > +.PP > +The following value may be bitwise ORed in > +.IR mode > +to change the behavior of the > +.B UFFDIO_CONTINUE > +operation: > +.TP > +.B UFFDIO_CONTINUE_MODE_DONTWAKE > +Do not wake up the thread that waits for page-fault resolution. > +.PP > +The > +.I mapped > +field is used by the kernel to return the number of bytes > +that were actually mapped, or an error in the same manner as > +.BR UFFDIO_COPY . > +If the value returned in the > +.I mapped > +field doesn't match the value that was specified in > +.IR range.len , > +the operation fails with the error > +.BR EAGAIN . > +The > +.I mapped > +field is output-only; > +it is not read by the > +.B UFFDIO_CONTINUE > +operation. > +.PP > +This > +.BR ioctl (2) > +operation returns 0 on success. > +In this case, the entire area was mapped. > +On error, \-1 is returned and > +.I errno > +is set to indicate the error. > +Possible errors include: > +.TP > +.B EAGAIN > +The number of bytes mapped (i.e., the value returned in the > +.I mapped > +field) does not equal the value that was specified in the > +.I range.len > +field. > +.TP > +.B EINVAL > +Either > +.I range.start > +or > +.I range.len > +was not a multiple of the system page size; or > +.I range.len > +was zero; or the range specified was invalid. > +.TP > +.B EINVAL > +An invalid bit was specified in the > +.IR mode > +field. > +.TP > +.B EEXIST > +One or more pages were already mapped in the given range. > +.TP > +.B ENOENT > +The faulting process has changed its virtual memory layout simultaneously with > +an outstanding > +.B UFFDIO_CONTINUE > +operation. > +.TP > +.B ENOMEM > +Allocating memory needed to setup the page table mappings failed. > +.TP > +.B EFAULT > +No existing page could be found in the page cache for the given range. > +.TP > +.BR ESRCH > +The faulting process has exited at the time of a > +.B UFFDIO_CONTINUE > +operation. > +.\" > .SH RETURN VALUE > See descriptions of the individual operations, above. > .SH ERRORS > diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 > index 593c189d8..07f53c6ff 100644 > --- a/man2/userfaultfd.2 > +++ b/man2/userfaultfd.2 > @@ -78,7 +78,7 @@ all memory ranges that were registered with the object are unregistered > and unread events are flushed. > .\" > .PP > -Userfaultfd supports two modes of registration: > +Userfaultfd supports three modes of registration: > .TP > .BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)" > When registered with > @@ -92,6 +92,18 @@ or an > .B UFFDIO_ZEROPAGE > ioctl. > .TP > +.BR UFFDIO_REGISTER_MODE_MINOR " (since 5.13)" > +When registered with > +.B UFFDIO_REGISTER_MODE_MINOR > +mode, user-space will receive a page-fault notification > +when a minor page fault occurs. > +That is, when a backing page is in the page cache, but > +page table entries don't yet exist. > +The faulted thread will be stopped from execution until the page fault is > +resolved from user-space by an > +.B UFFDIO_CONTINUE > +ioctl. > +.TP > .BR UFFDIO_REGISTER_MODE_WP " (since 5.7)" > When registered with > .B UFFDIO_REGISTER_MODE_WP > @@ -212,9 +224,10 @@ a page fault occurring in the requested memory range, and satisfying > the mode defined at the registration time, will be forwarded by the kernel to > the user-space application. > The application can then use the > -.B UFFDIO_COPY > +.B UFFDIO_COPY , > +.B UFFDIO_ZEROPAGE , > or > -.B UFFDIO_ZEROPAGE > +.B UFFDIO_CONTINUE > .BR ioctl (2) > operations to resolve the page fault. > .PP > @@ -318,6 +331,43 @@ should have the flag > cleared upon the faulted page or range. > .PP > Write-protect mode supports only private anonymous memory. > +.\" > +.SS Userfaultfd minor fault mode (since 5.13) > +Since Linux 5.13, userfaultfd supports minor fault mode. > +In this mode, fault messages are produced not for major faults (where the > +page was missing), but rather for minor faults, where a page exists in the page > +cache, but the page table entries are not yet present. > +The user needs to first check availability of this feature using > +.B UFFDIO_API > +ioctl against the feature bit > +.B UFFD_FEATURE_MINOR_HUGETLBFS > +before using this feature. > +.PP > +To register with userfaultfd minor fault mode, the user needs to initiate the > +.B UFFDIO_REGISTER > +ioctl with mode > +.B UFFD_REGISTER_MODE_MINOR > +set. > +.PP > +When a minor fault occurs, user-space will receive a page-fault notification > +whose > +.I uffd_msg.pagefault.flags > +will have the > +.B UFFD_PAGEFAULT_FLAG_MINOR > +flag set. > +.PP > +To resolve a minor page fault, the handler should decide whether or not the > +existing page contents need to be modified first. > +If so, this should be done in-place via a second, non-userfaultfd-registered > +mapping to the same backing page (e.g., by mapping the hugetlbfs file twice). > +Once the page is considered "up to date", the fault can be resolved by > +initiating an > +.B UFFDIO_CONTINUE > +ioctl, which installs the page table entries and (by default) wakes up the > +faulting thread(s). > +.PP > +Minor fault mode supports only hugetlbfs-backed memory. > +.\" > .SS Reading from the userfaultfd structure > Each > .BR read (2) > @@ -456,19 +506,20 @@ For > the following flag may appear: > .RS > .TP > -.B UFFD_PAGEFAULT_FLAG_WRITE > -If the address is in a range that was registered with the > -.B UFFDIO_REGISTER_MODE_MISSING > -flag (see > -.BR ioctl_userfaultfd (2)) > -and this flag is set, this a write fault; > -otherwise it is a read fault. > +.B UFFD_PAGEFAULT_FLAG_WP > +If this flag is set, then the fault was a write-protect fault. > .TP > +.B UFFD_PAGEFAULT_FLAG_MINOR > +If this flag is set, then the fault was a minor fault. > +.TP > +.B UFFD_PAGEFAULT_FLAG_WRITE > +If this flag is set, then the fault was a write fault. > +.HP > +If neither > .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 is a write-protect fault. > -Otherwise it is a page-missing fault. > +nor > +.B UFFD_PAGEFAULT_FLAG_MINOR > +are set, then the fault was a missing fault. > .RE > .TP > .I pagefault.feat.pid > -- > 2.32.0.rc1.229.g3e70b5a671-goog >