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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 4F643C04AB4 for ; Wed, 15 May 2019 02:55:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1EE7D2084A for ; Wed, 15 May 2019 02:55:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726412AbfEOCzo (ORCPT ); Tue, 14 May 2019 22:55:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58264 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726201AbfEOCzo (ORCPT ); Tue, 14 May 2019 22:55:44 -0400 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 mx1.redhat.com (Postfix) with ESMTPS id 97B25308123F; Wed, 15 May 2019 02:55:43 +0000 (UTC) Received: from [10.72.12.103] (ovpn-12-103.pek2.redhat.com [10.72.12.103]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3C576608A6; Wed, 15 May 2019 02:55:36 +0000 (UTC) Subject: Re: [PATCH net] vhost: don't use kmap() to log dirty pages To: "Michael S. Tsirkin" Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , James Bottomley , Andrea Arcangeli , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart References: <1557725265-63525-1-git-send-email-jasowang@redhat.com> <20190514181150-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Wed, 15 May 2019 10:55:35 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190514181150-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Wed, 15 May 2019 02:55:43 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/5/15 上午6:16, Michael S. Tsirkin wrote: > On Mon, May 13, 2019 at 01:27:45AM -0400, Jason Wang wrote: >> Vhost log dirty pages directly to a userspace bitmap through GUP and >> kmap_atomic() since kernel doesn't have a set_bit_to_user() >> helper. This will cause issues for the arch that has virtually tagged >> caches. The way to fix is to keep using userspace virtual >> address. Fortunately, futex has arch_futex_atomic_op_inuser() which >> could be used for setting a bit to user. >> >> Note there're several cases that futex helper can fail e.g a page >> fault or the arch that doesn't have the support. For those cases, a >> simplified get_user()/put_user() pair protected by a global mutex is >> provided as a fallback. The fallback may lead false positive that >> userspace may see more dirty pages. >> >> Cc: Christoph Hellwig >> Cc: James Bottomley >> Cc: Andrea Arcangeli >> Cc: Thomas Gleixner >> Cc: Ingo Molnar >> Cc: Peter Zijlstra >> Cc: Darren Hart >> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server") >> Signed-off-by: Jason Wang >> --- >> Changes from RFC V2: >> - drop GUP and provide get_user()/put_user() fallbacks >> - round down log_base >> Changes from RFC V1: >> - switch to use arch_futex_atomic_op_inuser() >> --- >> drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++----------------------- >> 1 file changed, 30 insertions(+), 24 deletions(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 351af88..7fa05ba 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -31,6 +31,7 @@ >> #include >> #include >> #include >> +#include >> >> #include "vhost.h" >> >> @@ -43,6 +44,8 @@ >> MODULE_PARM_DESC(max_iotlb_entries, >> "Maximum number of iotlb entries. (default: 2048)"); >> >> +static DEFINE_MUTEX(vhost_log_lock); >> + >> enum { >> VHOST_MEMORY_F_LOG = 0x1, >> }; >> @@ -1692,28 +1695,31 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp) >> } >> EXPORT_SYMBOL_GPL(vhost_dev_ioctl); >> >> -/* TODO: This is really inefficient. We need something like get_user() >> - * (instruction directly accesses the data, with an exception table entry >> - * returning -EFAULT). See Documentation/x86/exception-tables.txt. >> - */ >> -static int set_bit_to_user(int nr, void __user *addr) >> +static int set_bit_to_user(int nr, u32 __user *addr) >> { >> - unsigned long log = (unsigned long)addr; >> - struct page *page; >> - void *base; >> - int bit = nr + (log % PAGE_SIZE) * 8; >> + u32 old; >> int r; >> >> - r = get_user_pages_fast(log, 1, 1, &page); >> - if (r < 0) >> - return r; >> - BUG_ON(r != 1); >> - base = kmap_atomic(page); >> - set_bit(bit, base); >> - kunmap_atomic(base); >> - set_page_dirty_lock(page); >> - put_page(page); >> + r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr); >> + if (r) { >> + /* Fallback through get_user()/put_user(), this may >> + * lead false positive that userspace may see more >> + * dirty pages. A mutex is used to synchronize log >> + * access between vhost threads. >> + */ >> + mutex_lock(&vhost_log_lock); >> + r = get_user(old, addr); >> + if (r) >> + goto err; >> + r = put_user(old | 1 << nr, addr); >> + if (r) >> + goto err; >> + mutex_unlock(&vhost_log_lock); >> + } > Problem is, we always said it's atomic. > > This trick will work if userspace only clears bits > in the log, but won't if it sets bits in the log. > E.g. reusing the log structure for vhost-user > will have exactly this effect. > Ok, I admit this is an issue. Then I think maybe we can simply fallback to a u8 put_user() which is guaranteed to be atomic like: put_user(0xF, addr); Then we may at most have 7 more dirty pages to be seen by guest. Consider futex helper is implemented is most architectures and it's likely to succeed. It should be acceptable. Does this make sense? Thanks