From patchwork Wed Feb 10 21:21:50 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12081785 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=-11.6 required=3.0 tests=BAYES_00, DKIM_ADSP_CUSTOM_MED,DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham 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 AD1F3C433DB for ; Wed, 10 Feb 2021 21:22:09 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0797364ED0 for ; Wed, 10 Feb 2021 21:22:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0797364ED0 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 76AEF6B0005; Wed, 10 Feb 2021 16:22:08 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 71A166B0006; Wed, 10 Feb 2021 16:22:08 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 60BB16B006C; Wed, 10 Feb 2021 16:22:08 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0214.hostedemail.com [216.40.44.214]) by kanga.kvack.org (Postfix) with ESMTP id 43EFA6B0005 for ; Wed, 10 Feb 2021 16:22:08 -0500 (EST) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id F0BE218233674 for ; Wed, 10 Feb 2021 21:22:07 +0000 (UTC) X-FDA: 77803631094.03.4BBA435 Received: from mail-yb1-f201.google.com (mail-yb1-f201.google.com [209.85.219.201]) by imf13.hostedemail.com (Postfix) with ESMTP id 2C85DE0001B4 for ; Wed, 10 Feb 2021 21:22:07 +0000 (UTC) Received: by mail-yb1-f201.google.com with SMTP id k7so3882767ybm.13 for ; Wed, 10 Feb 2021 13:22:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:message-id:mime-version:subject:from:to:cc; bh=f3bDipLNxq+4vQj/7fDtE/PjMb4mWt44G3LXQ2qAmsY=; b=pQ2zUjlS/rzaBJwpjf02JILnvkXRckNWJVMgXVzbZ7nYG9888IQpy5wdTCuUEQifYw NZxwo2sBI7KZNBa2vv7HgAMK7vbpBJVJtQHWqXTfKfj0tRet+pPbDg88ps5sqy+6L16X nxADQpvCW3Sgv4kavvKUFLuyoJ3O8f5jUxl3O5crFBAIe/PHSfmAu1462Zo5FK73tDU6 QocJJ6sT0LKVxBRlmXe6dob19ruK7AgEQ6unhcI4tDPpyZrDBaxOvfoIsckW3jS/w0oQ Fu6fOR6EF/fgTqOm4CLBhLHJ9uAcAF/yp3Uv6KonYX8Z3jb2H2Hivo++hiJw8zj1U7w2 IYCQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:message-id:mime-version:subject:from :to:cc; bh=f3bDipLNxq+4vQj/7fDtE/PjMb4mWt44G3LXQ2qAmsY=; b=nUEzAyvvTfUUTBkLmzjyJogVXu8KHmThtmMhTd56GDAYaUgz03XhxV2gChSBGgd/a0 LWctsF94ziFDzebEPS8NZPEXqQdadUVi7JjTAHqcorvHT7jnpoD+zyYEKfxJ5P06GRlC TTl/9E56j+v2ECFgq39LMaAAO7g9gbtTaDnD5j0M05rq71NBADVO+2rDijgYCA8ZEsNQ oyJ5UlTCazCaNDSe9F1dIzUblfPOVqSEeWdFfssc6S8Oh/sKxn9CgMwbUW3IuJhierqM a+MHpkXpP9zmW0OKpLcvz9KmqbiGjiQk9YBxTTNiEh51QWtz2oxn8UlNArqhzodHMNiW 6aVw== X-Gm-Message-State: AOAM533ZGsxgRYHWcsPn6Te16jt5t5a24floKaSb/6KI4nA40LTwoEZa J879SC/Mt+8WGsyLe0CZjOLwFTVD3SClM3mELWQp X-Google-Smtp-Source: ABdhPJxeiA6mwLfMVFGQz1imqphRN1QNjEGKnuWaMfUI1pAt1XJAlZNkHz1V5p/SMS6DvK15jkqM4u6nwwGQiY3nMhvL X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:94ee:de01:168:9f20]) (user=axelrasmussen job=sendgmr) by 2002:a25:c090:: with SMTP id c138mr7134204ybf.314.1612992126484; Wed, 10 Feb 2021 13:22:06 -0800 (PST) Date: Wed, 10 Feb 2021 13:21:50 -0800 Message-Id: <20210210212200.1097784-1-axelrasmussen@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.30.0.478.g8a0d178c01-goog Subject: [PATCH v5 00/10] userfaultfd: add minor fault handling From: Axel Rasmussen To: Alexander Viro , Alexey Dobriyan , Andrea Arcangeli , Andrew Morton , Anshuman Khandual , Catalin Marinas , Chinwen Chang , Huang Ying , Ingo Molnar , Jann Horn , Jerome Glisse , Lokesh Gidra , "Matthew Wilcox (Oracle)" , Michael Ellerman , " =?utf-8?q?Michal_Koutn=C3=BD?= " , Michel Lespinasse , Mike Kravetz , Mike Rapoport , Nicholas Piggin , Peter Xu , Shaohua Li , Shawn Anastasio , Steven Rostedt , Steven Price , Vlastimil Babka Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Adam Ruprecht , Axel Rasmussen , Cannon Matthews , "Dr . David Alan Gilbert" , David Rientjes , Mina Almasry , Oliver Upton X-Stat-Signature: 3833c97dwgmgj378qpfqg3cjtz5mzuhm X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2C85DE0001B4 Received-SPF: none (flex--axelrasmussen.bounces.google.com>: No applicable sender policy available) receiver=imf13; identity=mailfrom; envelope-from="<3fk4kYA0KCLMTqXekTlfnllXgZhhZeX.Vhfebgnq-ffdoTVd.hkZ@flex--axelrasmussen.bounces.google.com>"; helo=mail-yb1-f201.google.com; client-ip=209.85.219.201 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1612992127-525761 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: Changelog ========= v4->v5: - Typo fix in the documentation update. - Removed comment in vma_can_userfault. The same information is better covered in the documentation update, so the comment is unnecessary (and slightly confusing as written). - Reworded comment for MCOPY_ATOMIC_CONTINUE mode. - For non-shared CONTINUE, only make the PTE(s) non-writable, don't change flags on the VMA. - In hugetlb_mcopy_atomic_pte, always unlock the page in MCOPY_ATOMIC_CONTINUE, even if we don't have VM_SHARED. - In hugetlb_mcopy_atomic_pte, introduce "bool is_continue" to make that kind of mode check more terse. - Merged two nested if()s into a single expression in __mcopy_atomic_hugetlb. - Moved "return -EINVAL if MCOPY_CONTINUE isn't supported for this vma type" up one level, into __mcopy_atomic. - Rebased onto linux-next/akpm, instead of the latest 5.11 RC. Resolved conflicts with Mike's recent hugetlb changes. v3->v4: - Relaxed restriction for minor registration to allow any hugetlb VMAs, not just those with VM_SHARED. Fixed setting VM_WRITE flag in a CONTINUE ioctl for non-VM_SHARED VMAs. - Reordered if() branches in hugetlb_mcopy_atomic_pte, so the conditions are simpler and easier to read. - Reverted most of the mfill_atomic_pte change (the anon / shmem path). Just return -EINVAL for CONTINUE, and set zeropage = (mode == MCOPY_ATOMIC_ZEROPAGE), so we can keep the delta small. - Split out adding #ifdef CONFIG_USERFAULTFD to a separate patch (instead of lumping it together with adding UFFDIO_CONTINUE). - Fixed signature of hugetlb_mcopy_atomic_pte for !CONFIG_HUGETLB_PAGE (signature must be the same in either case). - Rebased onto a newer version of Peter's patches to disable huge PMD sharing. v2->v3: - Added #ifdef CONFIG_USERFAULTFD around hugetlb helper functions, to fix build errors when building without CONFIG_USERFAULTFD set. v1->v2: - Fixed a bug in the hugetlb_mcopy_atomic_pte retry case. We now plumb in the enum mcopy_atomic_mode, so we can differentiate between the three cases this function needs to handle: 1) We're doing a COPY op, and need to allocate a page, add to cache, etc. 2) We're doing a COPY op, but allocation in this function failed previously; we're in the retry path. The page was allocated, but not e.g. added to page cache, so that still needs to be done. 3) We're doing a CONTINUE op, we need to look up an existing page instead of allocating a new one. - Rebased onto a newer version of Peter's patches to disable huge PMD sharing, which fixes syzbot complaints on some non-x86 architectures. - Moved __VM_UFFD_FLAGS into userfaultfd_k.h, so inline helpers can use it. - Renamed UFFD_FEATURE_MINOR_FAULT_HUGETLBFS to UFFD_FEATURE_MINOR_HUGETLBFS, for consistency with other existing feature flags. - Moved the userfaultfd_minor hook in hugetlb.c into the else block, so we don't have to explicitly check for !new_page. RFC->v1: - Rebased onto Peter Xu's patches for disabling huge PMD sharing for certain userfaultfd-registered areas. - Added commits which update documentation, and add a self test which exercises the new feature. - Fixed reporting CONTINUE as a supported ioctl even for non-MINOR ranges. Overview ======== This series adds a new userfaultfd registration mode, UFFDIO_REGISTER_MODE_MINOR. This allows userspace to intercept "minor" faults. By "minor" fault, I mean the following situation: Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory). One of the mappings is registered with userfaultfd (in minor mode), and the other is not. Via the non-UFFD mapping, the underlying pages have already been allocated & filled with some contents. The UFFD mapping has not yet been faulted in; when it is touched for the first time, this results in what I'm calling a "minor" fault. As a concrete example, when working with hugetlbfs, we have huge_pte_none(), but find_lock_page() finds an existing page. We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is, userspace resolves the fault by either a) doing nothing if the contents are already correct, or b) updating the underlying contents using the second, non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA, or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel "I have ensured the page contents are correct, carry on setting up the mapping". Use Case ======== Consider the use case of VM live migration (e.g. under QEMU/KVM): 1. While a VM is still running, we copy the contents of its memory to a target machine. The pages are populated on the target by writing to the non-UFFD mapping, using the setup described above. The VM is still running (and therefore its memory is likely changing), so this may be repeated several times, until we decide the target is "up to date enough". 2. We pause the VM on the source, and start executing on the target machine. During this gap, the VM's user(s) will *see* a pause, so it is desirable to minimize this window. 3. Between the last time any page was copied from the source to the target, and when the VM was paused, the contents of that page may have changed - and therefore the copy we have on the target machine is out of date. Although we can keep track of which pages are out of date, for VMs with large amounts of memory, it is "slow" to transfer this information to the target machine. We want to resume execution before such a transfer would complete. 4. So, the guest begins executing on the target machine. The first time it touches its memory (via the UFFD-registered mapping), userspace wants to intercept this fault. Userspace checks whether or not the page is up to date, and if not, copies the updated page from the source machine, via the non-UFFD mapping. Finally, whether a copy was performed or not, userspace issues a UFFDIO_CONTINUE ioctl to tell the kernel "I have ensured the page contents are correct, carry on setting up the mapping". We don't have to do all of the final updates on-demand. The userfaultfd manager can, in the background, also copy over updated pages once it receives the map of which pages are up-to-date or not. Interaction with Existing APIs ============================== Because it's possible to combine registration modes (e.g. a single VMA can be userfaultfd-registered MINOR | MISSING), and because it's up to userspace how to resolve faults once they are received, I spent some time thinking through how the existing API interacts with the new feature. UFFDIO_CONTINUE cannot be used to resolve non-minor faults, as it does not allocate a new page. If UFFDIO_CONTINUE is used on a non-minor fault: - For non-shared memory or shmem, -EINVAL is returned. - For hugetlb, -EFAULT is returned. UFFDIO_COPY and UFFDIO_ZEROPAGE cannot be used to resolve minor faults. Without modifications, the existing codepath assumes a new page needs to be allocated. This is okay, since userspace must have a second non-UFFD-registered mapping anyway, thus there isn't much reason to want to use these in any case (just memcpy or memset or similar). - If UFFDIO_COPY is used on a minor fault, -EEXIST is returned. - If UFFDIO_ZEROPAGE is used on a minor fault, -EEXIST is returned (or -EINVAL in the case of hugetlb, as UFFDIO_ZEROPAGE is unsupported in any case). - UFFDIO_WRITEPROTECT simply doesn't work with shared memory, and returns -ENOENT in that case (regardless of the kind of fault). Dependencies ============ I've included 4 commits from Peter Xu's larger series (https://lore.kernel.org/patchwork/cover/1366017/) in this series. My changes depend on his work, to disable huge PMD sharing for MINOR registered userfaultfd areas. I included the 4 commits directly because a) it lets this series just be applied and work as-is, and b) they are fairly standalone, and could potentially be merged even without the rest of the larger series Peter submitted. Thanks Peter! Also, although it doesn't affect minor fault handling, I did notice that the userfaultfd self test sometimes experienced memory corruption (https://lore.kernel.org/patchwork/cover/1356755/). For anyone testing this series, it may be useful to apply that series first to fix the selftest flakiness. That series doesn't have to be merged into mainline / maintaner branches before mine, though. Future Work =========== Currently the patchset only supports hugetlbfs. There is no reason it can't work with shmem, but I expect hugetlbfs to be much more commonly used since we're talking about backing guest memory for VMs. I plan to implement shmem support in a follow-up patch series. Axel Rasmussen (6): userfaultfd: add minor fault registration mode userfaultfd: disable huge PMD sharing for MINOR registered VMAs userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled userfaultfd: add UFFDIO_CONTINUE ioctl userfaultfd: update documentation to describe minor fault handling userfaultfd/selftests: add test exercising minor fault handling Peter Xu (4): hugetlb: Pass vma into huge_pte_alloc() and huge_pmd_share() hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled mm/hugetlb: Move flush_hugetlb_tlb_range() into hugetlb.h hugetlb/userfaultfd: Unshare all pmds for hugetlbfs when register wp Documentation/admin-guide/mm/userfaultfd.rst | 107 +++++++---- arch/arm64/mm/hugetlbpage.c | 7 +- arch/ia64/mm/hugetlbpage.c | 3 +- arch/mips/mm/hugetlbpage.c | 4 +- arch/parisc/mm/hugetlbpage.c | 2 +- arch/powerpc/mm/hugetlbpage.c | 3 +- arch/s390/mm/hugetlbpage.c | 2 +- arch/sh/mm/hugetlbpage.c | 2 +- arch/sparc/mm/hugetlbpage.c | 6 +- fs/proc/task_mmu.c | 1 + fs/userfaultfd.c | 186 +++++++++++++++---- include/linux/hugetlb.h | 22 ++- include/linux/mm.h | 1 + include/linux/mmu_notifier.h | 1 + include/linux/userfaultfd_k.h | 49 ++++- include/trace/events/mmflags.h | 1 + include/uapi/linux/userfaultfd.h | 36 +++- mm/hugetlb.c | 116 ++++++++---- mm/userfaultfd.c | 39 ++-- tools/testing/selftests/vm/userfaultfd.c | 147 ++++++++++++++- 20 files changed, 587 insertions(+), 148 deletions(-) --- 2.30.0.478.g8a0d178c01-goog