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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 A69BFC432C3 for ; Wed, 13 Nov 2019 18:47:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E0219206F6 for ; Wed, 13 Nov 2019 18:47:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727735AbfKMSr6 (ORCPT ); Wed, 13 Nov 2019 13:47:58 -0500 Received: from mga17.intel.com ([192.55.52.151]:43379 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727241AbfKMSr6 (ORCPT ); Wed, 13 Nov 2019 13:47:58 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 13 Nov 2019 10:47:57 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,301,1569308400"; d="scan'208";a="216489509" Received: from iweiny-desk2.sc.intel.com ([10.3.52.157]) by orsmga002.jf.intel.com with ESMTP; 13 Nov 2019 10:47:53 -0800 Date: Wed, 13 Nov 2019 10:47:53 -0800 From: Ira Weiny To: John Hubbard Cc: Andrew Morton , Al Viro , Alex Williamson , Benjamin Herrenschmidt , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , Christoph Hellwig , Dan Williams , Daniel Vetter , Dave Chinner , David Airlie , "David S . Miller" , Jan Kara , Jason Gunthorpe , Jens Axboe , Jonathan Corbet , =?iso-8859-1?B?Suly9G1l?= Glisse , Magnus Karlsson , Mauro Carvalho Chehab , Michael Ellerman , Michal Hocko , Mike Kravetz , Paul Mackerras , Shuah Khan , Vlastimil Babka , bpf@vger.kernel.org, dri-devel@lists.freedesktop.org, kvm@vger.kernel.org, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org, linux-rdma@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-mm@kvack.org, LKML Subject: Re: [PATCH v4 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM Message-ID: <20191113184752.GD12699@iweiny-DESK2.sc.intel.com> References: <20191113042710.3997854-1-jhubbard@nvidia.com> <20191113042710.3997854-9-jhubbard@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191113042710.3997854-9-jhubbard@nvidia.com> User-Agent: Mutt/1.11.1 (2018-12-01) Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Nov 12, 2019 at 08:26:55PM -0800, John Hubbard wrote: > As it says in the updated comment in gup.c: current FOLL_LONGTERM > behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the > FS DAX check requirement on vmas. > > However, the corresponding restriction in get_user_pages_remote() was > slightly stricter than is actually required: it forbade all > FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers > that do not set the "locked" arg. > > Update the code and comments accordingly, and update the VFIO caller > to take advantage of this, fixing a bug as a result: the VFIO caller > is logically a FOLL_LONGTERM user. > > Also, remove an unnessary pair of calls that were releasing and > reacquiring the mmap_sem. There is no need to avoid holding mmap_sem > just in order to call page_to_pfn(). > > Also, move the DAX check ("if a VMA is DAX, don't allow long term > pinning") from the VFIO call site, all the way into the internals > of get_user_pages_remote() and __gup_longterm_locked(). That is: > get_user_pages_remote() calls __gup_longterm_locked(), which in turn > calls check_dax_vmas(). It's lightly explained in the comments as well. > > Thanks to Jason Gunthorpe for pointing out a clean way to fix this, > and to Dan Williams for helping clarify the DAX refactoring. > > Suggested-by: Jason Gunthorpe > Cc: Dan Williams > Cc: Jerome Glisse > Cc: Ira Weiny Reviewed-by: Ira Weiny > Signed-off-by: John Hubbard > --- > drivers/vfio/vfio_iommu_type1.c | 25 ++----------------------- > mm/gup.c | 27 ++++++++++++++++++++++----- > 2 files changed, 24 insertions(+), 28 deletions(-) > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > index d864277ea16f..7301b710c9a4 100644 > --- a/drivers/vfio/vfio_iommu_type1.c > +++ b/drivers/vfio/vfio_iommu_type1.c > @@ -340,7 +340,6 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, > { > struct page *page[1]; > struct vm_area_struct *vma; > - struct vm_area_struct *vmas[1]; > unsigned int flags = 0; > int ret; > > @@ -348,33 +347,13 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, > flags |= FOLL_WRITE; > > down_read(&mm->mmap_sem); > - if (mm == current->mm) { > - ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page, > - vmas); > - } else { > - ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page, > - vmas, NULL); > - /* > - * The lifetime of a vaddr_get_pfn() page pin is > - * userspace-controlled. In the fs-dax case this could > - * lead to indefinite stalls in filesystem operations. > - * Disallow attempts to pin fs-dax pages via this > - * interface. > - */ > - if (ret > 0 && vma_is_fsdax(vmas[0])) { > - ret = -EOPNOTSUPP; > - put_page(page[0]); > - } > - } > - up_read(&mm->mmap_sem); > - > + ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM, > + page, NULL, NULL); > if (ret == 1) { > *pfn = page_to_pfn(page[0]); > return 0; > } > > - down_read(&mm->mmap_sem); > - > vaddr = untagged_addr(vaddr); > > vma = find_vma_intersection(mm, vaddr, vaddr + 1); > diff --git a/mm/gup.c b/mm/gup.c > index 933524de6249..83702b2e86c8 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -29,6 +29,13 @@ struct follow_page_context { > unsigned int page_mask; > }; > > +static __always_inline long __gup_longterm_locked(struct task_struct *tsk, > + struct mm_struct *mm, > + unsigned long start, > + unsigned long nr_pages, > + struct page **pages, > + struct vm_area_struct **vmas, > + unsigned int flags); > /* > * Return the compound head page with ref appropriately incremented, > * or NULL if that failed. > @@ -1167,13 +1174,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm, > struct vm_area_struct **vmas, int *locked) > { > /* > - * FIXME: Current FOLL_LONGTERM behavior is incompatible with > + * Parts of FOLL_LONGTERM behavior are incompatible with > * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on > - * vmas. As there are no users of this flag in this call we simply > - * disallow this option for now. > + * vmas. However, this only comes up if locked is set, and there are > + * callers that do request FOLL_LONGTERM, but do not set locked. So, > + * allow what we can. > */ > - if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM)) > - return -EINVAL; > + if (gup_flags & FOLL_LONGTERM) { > + if (WARN_ON_ONCE(locked)) > + return -EINVAL; > + /* > + * This will check the vmas (even if our vmas arg is NULL) > + * and return -ENOTSUPP if DAX isn't allowed in this case: > + */ > + return __gup_longterm_locked(tsk, mm, start, nr_pages, pages, > + vmas, gup_flags | FOLL_TOUCH | > + FOLL_REMOTE); > + } > > return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas, > locked, > -- > 2.24.0 >