From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754470Ab2E1NAk (ORCPT ); Mon, 28 May 2012 09:00:40 -0400 Received: from e28smtp08.in.ibm.com ([122.248.162.8]:35140 "EHLO e28smtp08.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753117Ab2E1NAj (ORCPT ); Mon, 28 May 2012 09:00:39 -0400 Message-ID: <4FC37600.1060301@linux.vnet.ibm.com> Date: Mon, 28 May 2012 20:56:32 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 MIME-Version: 1.0 To: Avi Kivity CC: Marcelo Tosatti , LKML , KVM , Andrea Arcangeli Subject: Re: [PATCH] KVM: MMU: fix huge page adapted on non-PAE host References: <4FC316E3.6080607@linux.vnet.ibm.com> <4FC35A15.6080000@redhat.com> <4FC363EE.6060204@linux.vnet.ibm.com> <4FC36E85.4010909@redhat.com> In-Reply-To: <4FC36E85.4010909@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit x-cbid: 12052812-2000-0000-0000-000007B94C89 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/28/2012 08:24 PM, Avi Kivity wrote: > On 05/28/2012 02:39 PM, Xiao Guangrong wrote: >> On 05/28/2012 06:57 PM, Avi Kivity wrote: >> >>> On 05/28/2012 09:10 AM, Xiao Guangrong wrote: >>>> The huge page size is 4M on non-PAE host, but 2M page size is used in >>>> transparent_hugepage_adjust(), so the page we get after adjust the >>>> mapping level is not the head page, the BUG_ON() will be triggered >>>> >>>> >>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c >>>> index 72102e0..be3cea4 100644 >>>> --- a/arch/x86/kvm/mmu.c >>>> +++ b/arch/x86/kvm/mmu.c >>>> @@ -2595,8 +2595,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu, >>>> *gfnp = gfn; >>>> kvm_release_pfn_clean(pfn); >>>> pfn &= ~mask; >>>> - if (!get_page_unless_zero(pfn_to_page(pfn))) >>>> - BUG(); >>>> + kvm_get_pfn(pfn); >>>> *pfnp = pfn; >>>> } >>>> } >>> >>> Shouldn't we adjust mask instead? >>> >> >> >> Adjusting mask to map the whole 4M huge page to KVM guest? > > The code moves the refcount from the small page to the huge page. i.e. > from pfn 0x1312 to pfn 0x1200. But if the huge page frame contains > 0x400 pages, it should move the refcount to pfn 0x1000. > We need not move the refcount to the huge page (the head of pages), moving the refcount to the any middle small page is also ok, get_page() will properly handle it: get_page() -> __get_page_tail(): | struct page *page_head = compound_trans_head(page); | | if (likely(page != page_head && get_page_unless_zero(page_head))) { | /* | * page_head wasn't a dangling pointer but it | * may not be a head page anymore by the time | * we obtain the lock. That is ok as long as it | * can't be freed from under us. | */ | flags = compound_lock_irqsave(page_head); | /* here __split_huge_page_refcount won't run anymore */ | if (likely(PageTail(page))) { | __get_page_tail_foll(page, false); | got = true; | } | compound_unlock_irqrestore(page_head, flags); | if (unlikely(!got)) | put_page(page_head); | } The refcount of page_head is increased. >> But it seams 4M page size is not supported on VMX/SVM. > > We always use 64-bit PTEs in the lowest level, whether using shadow, > EPT, or NPT. Note NPT supports 32-bit PTEs in the lowest level, but we > don't support that configuration. But that doesn't mean we can't use > host 4M pages to back guest 2M pages (or direct maps). > Right.