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=-6.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 3DF91C04A6B for ; Wed, 8 May 2019 04:28:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 18EAA21019 for ; Wed, 8 May 2019 04:28:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727087AbfEHE2v (ORCPT ); Wed, 8 May 2019 00:28:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33908 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725825AbfEHE2v (ORCPT ); Wed, 8 May 2019 00:28:51 -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 33DDB307D987; Wed, 8 May 2019 04:28:51 +0000 (UTC) Received: from [10.72.12.176] (ovpn-12-176.pek2.redhat.com [10.72.12.176]) by smtp.corp.redhat.com (Postfix) with ESMTP id 650282E041; Wed, 8 May 2019 04:28:44 +0000 (UTC) Subject: Re: [PATCH RFC] 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: <1557195809-12373-1-git-send-email-jasowang@redhat.com> <20190507220526-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <84a2237e-d8cf-922c-0d0b-90009e99e8ee@redhat.com> Date: Wed, 8 May 2019 12:28:42 +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: <20190507220526-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 [10.5.110.48]); Wed, 08 May 2019 04:28:51 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2019/5/8 下午12:12, Michael S. Tsirkin wrote: > On Mon, May 06, 2019 at 10:23:29PM -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 a cmpxchg to userspace memory helper >> futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the >> userspace bitmap with zero, set the bit and then write it back through >> put_user(). >> >> Note: there're archs (few non popular ones) that don't implement >> futex helper, we can't log dirty pages. We can fix them on top or >> simply disable LOG_ALL features of vhost. > Or implement futex_atomic_cmpxchg using kmap if they don't have > virtually tagged caches. Yes, this might work. > >> Cc: Christoph Hellwig >> Cc: James Bottomley >> Cc: Andrea Arcangeli >> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server") >> Signed-off-by: Jason Wang >> --- >> drivers/vhost/vhost.c | 27 +++++++++++++++------------ >> 1 file changed, 15 insertions(+), 12 deletions(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 351af88..9c94c41 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -31,6 +31,7 @@ >> #include >> #include >> #include >> +#include >> >> #include "vhost.h" >> >> @@ -1692,25 +1693,27 @@ 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_log; >> 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); >> + >> + r = futex_atomic_cmpxchg_inatomic(&old_log, addr, 0, 0); >> + if (r < 0) >> + return r; > So I think this is a great idea! > > However one issue here is that futex_atomic_cmpxchg_inatomic will fail if the > page is swapped out. I suspect we need a variant that blocks the thread > instead. I guess not since the patch still try to pin the page before. Thanks > >> + >> + old_log |= 1 << nr; >> + r = put_user(old_log, addr); >> + if (r < 0) >> + return r; >> + >> set_page_dirty_lock(page); >> put_page(page); >> return 0; >> @@ -1727,8 +1730,8 @@ static int log_write(void __user *log_base, >> write_length += write_address % VHOST_PAGE_SIZE; >> for (;;) { >> u64 base = (u64)(unsigned long)log_base; >> - u64 log = base + write_page / 8; >> - int bit = write_page % 8; >> + u64 log = base + write_page / 32; >> + int bit = write_page % 32; >> if ((u64)(unsigned long)log != log) >> return -EFAULT; >> r = set_bit_to_user(bit, (void __user *)(unsigned long)log); >> -- >> 1.8.3.1