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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 880A6C3A5A4 for ; Wed, 28 Aug 2019 15:55:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 63E45208CB for ; Wed, 28 Aug 2019 15:55:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726706AbfH1PzE (ORCPT ); Wed, 28 Aug 2019 11:55:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59943 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726410AbfH1PzE (ORCPT ); Wed, 28 Aug 2019 11:55:04 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 97C663007C30; Wed, 28 Aug 2019 15:55:03 +0000 (UTC) Received: from x1.home (ovpn-116-131.phx2.redhat.com [10.3.116.131]) by smtp.corp.redhat.com (Postfix) with ESMTP id C30A31001B14; Wed, 28 Aug 2019 15:55:02 +0000 (UTC) Date: Wed, 28 Aug 2019 09:55:01 -0600 From: Alex Williamson To: Ben Luo Cc: cohuck@redhat.com, linux-kernel@vger.kernel.org, Andrea Arcangeli Subject: Re: [PATCH v2] vfio/type1: avoid redundant PageReserved checking Message-ID: <20190828095501.12e71bd3@x1.home> In-Reply-To: <3517844d6371794cff59b13bf9c2baf1dcbe571c.1566966365.git.luoben@linux.alibaba.com> References: <20190827124041.4f986005@x1.home> <3517844d6371794cff59b13bf9c2baf1dcbe571c.1566966365.git.luoben@linux.alibaba.com> Organization: Red Hat MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Wed, 28 Aug 2019 15:55:03 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 28 Aug 2019 12:28:04 +0800 Ben Luo wrote: > currently, if the page is not a tail of compound page, it will be > checked twice for the same thing. > > Signed-off-by: Ben Luo > --- > drivers/vfio/vfio_iommu_type1.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > index 054391f..d0f7346 100644 > --- a/drivers/vfio/vfio_iommu_type1.c > +++ b/drivers/vfio/vfio_iommu_type1.c > @@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async) > static bool is_invalid_reserved_pfn(unsigned long pfn) > { > if (pfn_valid(pfn)) { > - bool reserved; > struct page *tail = pfn_to_page(pfn); > struct page *head = compound_head(tail); > - reserved = !!(PageReserved(head)); > if (head != tail) { > + bool reserved = PageReserved(head); > /* > * "head" is not a dangling pointer > * (compound_head takes care of that) Thinking more about this, the code here was originally just a copy of kvm_is_mmio_pfn() which was simplified in v3.12 with the commit below. Should we instead do the same thing here? Thanks, Alex commit 11feeb498086a3a5907b8148bdf1786a9b18fc55 Author: Andrea Arcangeli Date: Thu Jul 25 03:04:38 2013 +0200 kvm: optimize away THP checks in kvm_is_mmio_pfn() The checks on PG_reserved in the page structure on head and tail pages aren't necessary because split_huge_page wouldn't transfer the PG_reserved bit from head to tail anyway. This was a forward-thinking check done in the case PageReserved was set by a driver-owned page mapped in userland with something like remap_pfn_range in a VM_PFNMAP region, but using hugepmds (not possible right now). It was meant to be very safe, but it's overkill as it's unlikely split_huge_page could ever run without the driver noticing and tearing down the hugepage itself. And if a driver in the future will really want to map a reserved hugepage in userland using an huge pmd it should simply take care of marking all subpages reserved too to keep KVM safe. This of course would require such a hypothetical driver to tear down the huge pmd itself and splitting the hugepage itself, instead of relaying on split_huge_page, but that sounds very reasonable, especially considering split_huge_page wouldn't currently transfer the reserved bit anyway. Signed-off-by: Andrea Arcangeli Signed-off-by: Gleb Natapov diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index d2836788561e..0fc25aed79a8 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -102,28 +102,8 @@ static bool largepages_enabled = true; bool kvm_is_mmio_pfn(pfn_t pfn) { - if (pfn_valid(pfn)) { - int reserved; - struct page *tail = pfn_to_page(pfn); - struct page *head = compound_trans_head(tail); - reserved = PageReserved(head); - if (head != tail) { - /* - * "head" is not a dangling pointer - * (compound_trans_head takes care of that) - * but the hugepage may have been splitted - * from under us (and we may not hold a - * reference count on the head page so it can - * be reused before we run PageReferenced), so - * we've to check PageTail before returning - * what we just read. - */ - smp_rmb(); - if (PageTail(tail)) - return reserved; - } - return PageReserved(tail); - } + if (pfn_valid(pfn)) + return PageReserved(pfn_to_page(pfn)); return true; }