From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753125Ab2E1NOH (ORCPT ); Mon, 28 May 2012 09:14:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:20685 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751387Ab2E1NOE (ORCPT ); Mon, 28 May 2012 09:14:04 -0400 Message-ID: <4FC37A18.10809@redhat.com> Date: Mon, 28 May 2012 16:14:00 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 MIME-Version: 1.0 To: Xiao Guangrong 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> <4FC37600.1060301@linux.vnet.ibm.com> In-Reply-To: <4FC37600.1060301@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/28/2012 03:56 PM, Xiao Guangrong wrote: > 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. > So, the whole thing is unneeded? Andrea? -- error compiling committee.c: too many arguments to function