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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 F40CDC4332D for ; Tue, 9 Mar 2021 20:11:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BB97B652AD for ; Tue, 9 Mar 2021 20:11:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231550AbhCIUL0 (ORCPT ); Tue, 9 Mar 2021 15:11:26 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:37132 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230431AbhCIULO (ORCPT ); Tue, 9 Mar 2021 15:11:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615320673; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=pPWSIVWYD5uhXg5OS/brIRidglvKzUiZYDWWxI4gaXY=; b=cmnnVNEYewIYDygBI1gR7+zj+b3KcDL6mVRTArc8SO/Lf7DP9Jwkp0wXLXaqtVSD2iwike x5xB3CQf6BA0u0AzTF5UMXwrVAzL7KWYi3+72OXePejvt0XaU/pTS2xFXlbqXa3O42pDZo N3PFyJfGHTHpm8epdGRjyjmmBLSooVU= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-63-lQKzdEJdM7KaXRfMynlepw-1; Tue, 09 Mar 2021 15:11:11 -0500 X-MC-Unique: lQKzdEJdM7KaXRfMynlepw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 865CD801817; Tue, 9 Mar 2021 20:11:09 +0000 (UTC) Received: from omen.home.shazbot.org (ovpn-112-255.phx2.redhat.com [10.3.112.255]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3E5B360CCD; Tue, 9 Mar 2021 20:11:05 +0000 (UTC) Date: Tue, 9 Mar 2021 13:11:04 -0700 From: Alex Williamson To: Peter Xu Cc: Jason Gunthorpe , "Zengtao (B)" , Cornelia Huck , Kevin Tian , Andrew Morton , Giovanni Cabiddu , Michel Lespinasse , Jann Horn , Max Gurtovoy , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Linuxarm Subject: Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant Message-ID: <20210309131104.1094b798@omen.home.shazbot.org> In-Reply-To: <20210309194824.GE763132@xz-x1> References: <1615201890-887-1-git-send-email-prime.zeng@hisilicon.com> <20210308132106.49da42e2@omen.home.shazbot.org> <20210308225626.GN397383@xz-x1> <6b98461600f74f2385b9096203fa3611@hisilicon.com> <20210309124609.GG2356281@nvidia.com> <20210309082951.75f0eb01@x1.home.shazbot.org> <20210309164004.GJ2356281@nvidia.com> <20210309184739.GD763132@xz-x1> <20210309122607.0b68fb9b@omen.home.shazbot.org> <20210309194824.GE763132@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 9 Mar 2021 14:48:24 -0500 Peter Xu wrote: > On Tue, Mar 09, 2021 at 12:26:07PM -0700, Alex Williamson wrote: > > On Tue, 9 Mar 2021 13:47:39 -0500 > > Peter Xu wrote: > > > > > On Tue, Mar 09, 2021 at 12:40:04PM -0400, Jason Gunthorpe wrote: > > > > On Tue, Mar 09, 2021 at 08:29:51AM -0700, Alex Williamson wrote: > > > > > On Tue, 9 Mar 2021 08:46:09 -0400 > > > > > Jason Gunthorpe wrote: > > > > > > > > > > > On Tue, Mar 09, 2021 at 03:49:09AM +0000, Zengtao (B) wrote: > > > > > > > Hi guys: > > > > > > > > > > > > > > Thanks for the helpful comments, after rethinking the issue, I have proposed > > > > > > > the following change: > > > > > > > 1. follow_pte instead of follow_pfn. > > > > > > > > > > > > Still no on follow_pfn, you don't need it once you use vmf_insert_pfn > > > > > > > > > > vmf_insert_pfn() only solves the BUG_ON, follow_pte() is being used > > > > > here to determine whether the translation is already present to avoid > > > > > both duplicate work in inserting the translation and allocating a > > > > > duplicate vma tracking structure. > > > > > > > > Oh.. Doing something stateful in fault is not nice at all > > > > > > > > I would rather see __vfio_pci_add_vma() search the vma_list for dups > > > > than call follow_pfn/pte.. > > > > > > It seems to me that searching vma list is still the simplest way to fix the > > > problem for the current code base. I see io_remap_pfn_range() is also used in > > > the new series - maybe that'll need to be moved to where PCI_COMMAND_MEMORY got > > > turned on/off in the new series (I just noticed remap_pfn_range modifies vma > > > flags..), as you suggested in the other email. > > > > > > In the new series, I think the fault handler becomes (untested): > > > > static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf) > > { > > struct vm_area_struct *vma = vmf->vma; > > struct vfio_pci_device *vdev = vma->vm_private_data; > > unsigned long base_pfn, pgoff; > > vm_fault_t ret = VM_FAULT_SIGBUS; > > > > if (vfio_pci_bar_vma_to_pfn(vma, &base_pfn)) > > return ret; > > > > pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT; > > > > down_read(&vdev->memory_lock); > > > > if (__vfio_pci_memory_enabled(vdev)) > > ret = vmf_insert_pfn(vma, vmf->address, pgoff + base_pfn); > > > > up_read(&vdev->memory_lock); > > > > return ret; > > } > > It's just that the initial MMIO access delay would be spread to the 1st access > of each mmio page access rather than using the previous pre-fault scheme. I > think an userspace cares the delay enough should pre-fault all pages anyway, > but just raise this up. Otherwise looks sane. Yep, this is a concern. Is it safe to have loops concurrently and fully populating the same vma with vmf_insert_pfn()? If it is then we could just ignore that we're doing duplicate work when we hit this race condition. Otherwise we'd need to serialize again, perhaps via a lock and flag stored in a struct linked from vm_private_data, along with tracking to free that object :-\ Thanks, Alex