From patchwork Tue Mar 22 16:39:44 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12788823 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 05CCCC433EF for ; Tue, 22 Mar 2022 16:39:49 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 74FA66B0078; Tue, 22 Mar 2022 12:39:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 6FE806B007B; Tue, 22 Mar 2022 12:39:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5C5DB6B007D; Tue, 22 Mar 2022 12:39:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0056.hostedemail.com [216.40.44.56]) by kanga.kvack.org (Postfix) with ESMTP id 45B306B0078 for ; Tue, 22 Mar 2022 12:39:49 -0400 (EDT) Received: from smtpin31.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 01DB89F87D for ; Tue, 22 Mar 2022 16:39:49 +0000 (UTC) X-FDA: 79272583698.31.E383DE1 Received: from mail-yb1-f201.google.com (mail-yb1-f201.google.com [209.85.219.201]) by imf15.hostedemail.com (Postfix) with ESMTP id 76C73A002A for ; Tue, 22 Mar 2022 16:39:48 +0000 (UTC) Received: by mail-yb1-f201.google.com with SMTP id x9-20020a5b0809000000b00631d9edfb96so14813884ybp.22 for ; Tue, 22 Mar 2022 09:39:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:message-id:mime-version:subject:from:to:cc; bh=glY7LGDPCM25y+XbPq+POO1HsxgAlNCphRe/8HqyL+Q=; b=fLOMxDkbkAw8OxToVulHaPlTSEDnbye3Jw7uOlqRTsOgKI0kajSg2Lk5VHGkOtcrq0 26+j2/Sz1TowWjzP0ld76hLfdjOwJTkDDZvwJzryewJz/59ktD/g8Hcc/SaPABRe1HQv k65ocJ00udPPcXZxhN8MQS7t99zMHVD2aXuEvrd6lOIeO8Nw1NACsGbSUz7UP8AI9OCU SCjNlZmNgq3iWSknAmZfoR/UO78Vzgo/WBaloQYvz93hO0MHlZHdbkqWjECqJOFyCkE7 Gk23n2t03iIWol1Q6/vYBUYk3Zd8sAKPVdT/L6RDdxXEFnL0JRGfYzHBqsZzJFnNVR7t nJGw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=glY7LGDPCM25y+XbPq+POO1HsxgAlNCphRe/8HqyL+Q=; b=IMQlQ9zLfICWrrBiw6j5cG2hic7jAlo05T5aHM5k8DKGIql2Sr8gIk1JH0P02WAUVF HqttPgXosbulJ/Bed+S1B7uJQ/sgzxxOCbEN4240KEetYoqbLIiFZvgCXQGpo83C79Dv mMCa4Hr4lcMfgm+AXLhsMSfgU2I60N80eUpQPU5oHQS18bebHZcf39ExHlzVFPTmy2EF LAsLcmxMNCl4PPpJ//EuKO0NQthsVS7oUcZ3APh2cWk274Z7XcyroaoteiFhnii3B1cO e8oI6bFdgLFjse9B9HNZ8BnewzjMfFhvJdgn13OQ0oK+2ryv8aEBF6ZZN+jLtWKNVgud LVqQ== X-Gm-Message-State: AOAM531dExtCP3c3aUfmYZieJfx66goUQtlvbqHekL7zwr1McWnfC08S Xu1NH0iIBySqv5pvMQH4x+eI1aDfOhikcfG/0lc9 X-Google-Smtp-Source: ABdhPJzp+ZMbK3Vma/ovpf1hnlzQ5Doq7pv46l6RTTf2zuMM6G461HHoXoaLdSC328O+IIs5Y61WOu54RCXWcPtGpj+b X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:123f:9f6d:add4:8fce]) (user=axelrasmussen job=sendgmr) by 2002:a5b:848:0:b0:633:716f:1fb0 with SMTP id v8-20020a5b0848000000b00633716f1fb0mr25788968ybq.522.1647967187423; Tue, 22 Mar 2022 09:39:47 -0700 (PDT) Date: Tue, 22 Mar 2022 09:39:44 -0700 Message-Id: <20220322163944.631042-1-axelrasmussen@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.35.1.894.gb6a874cedc-goog Subject: [PATCH v3] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode From: Axel Rasmussen To: Alejandro Colomar , Andrea Arcangeli , Andrew Morton , Hugh Dickins , Michael Kerrisk , Mike Kravetz , Peter Xu Cc: linux-kernel@vger.kernel.org, linux-man@vger.kernel.org, linux-mm@kvack.org, Axel Rasmussen X-Rspamd-Queue-Id: 76C73A002A X-Rspam-User: Authentication-Results: imf15.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=fLOMxDkb; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf15.hostedemail.com: domain of 30_s5Yg0KCMoqDu17q82A88u3w44w1u.s421y3AD-220Bqs0.47w@flex--axelrasmussen.bounces.google.com designates 209.85.219.201 as permitted sender) smtp.mailfrom=30_s5Yg0KCMoqDu17q82A88u3w44w1u.s421y3AD-220Bqs0.47w@flex--axelrasmussen.bounces.google.com X-Stat-Signature: satj3n4k7oyg83sur9febfuauyhweaza X-Rspamd-Server: rspam07 X-HE-Tag: 1647967188-130431 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: 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. Reviewed-by: Peter Xu Signed-off-by: Axel Rasmussen --- man2/ioctl_userfaultfd.2 | 135 ++++++++++++++++++++++++++++++++++++--- man2/userfaultfd.2 | 79 +++++++++++++++++++---- 2 files changed, 192 insertions(+), 22 deletions(-) diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2 index 504f61d4b..d213a0a43 100644 --- a/man2/ioctl_userfaultfd.2 +++ b/man2/ioctl_userfaultfd.2 @@ -214,6 +214,11 @@ 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 +245,11 @@ 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 +288,8 @@ 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 +328,20 @@ 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 +750,110 @@ 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 cee7c01d2..458e05faa 100644 --- a/man2/userfaultfd.2 +++ b/man2/userfaultfd.2 @@ -82,7 +82,7 @@ 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 @@ -96,6 +96,18 @@ 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 @@ -216,9 +228,10 @@ 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 @@ -322,6 +335,43 @@ 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) @@ -460,19 +510,20 @@ 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. +.PP +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