From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab Date: Thu, 24 Oct 2013 19:01:49 +0800 Message-ID: <5268FE1D.50809@gmail.com> References: <1382534973-13197-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1382534973-13197-11-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <20131024091905.GB5289@redhat.com> <5268E888.7060608@linux.vnet.ibm.com> <20131024095204.GD5289@redhat.com> <5268F226.4030309@gmail.com> <20131024103925.GE5289@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Xiao Guangrong , avi.kivity@gmail.com, mtosatti@redhat.com, pbonzini@redhat.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org To: Gleb Natapov Return-path: In-Reply-To: <20131024103925.GE5289@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 10/24/2013 06:39 PM, Gleb Natapov wrote: > On Thu, Oct 24, 2013 at 06:10:46PM +0800, Xiao Guangrong wrote: >> On 10/24/2013 05:52 PM, Gleb Natapov wrote: >>> On Thu, Oct 24, 2013 at 05:29:44PM +0800, Xiao Guangrong wrote: >>>> On 10/24/2013 05:19 PM, Gleb Natapov wrote: >>>> >>>>>> @@ -946,7 +947,7 @@ static inline struct kvm_mmu_page *page_header(hpa_t shadow_page) >>>>>> { >>>>>> struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT); >>>>>> >>>>>> - return (struct kvm_mmu_page *)page_private(page); >>>>>> + return (struct kvm_mmu_page *)(page->mapping); >>>>> Why? >>>> >>>> That's because page->private has been used by slab: >>>> >>> But does lockless path actually looks at it? >> >> Lockless path does not use it, however, it is used by kvm_mmu_page(): >> >> static inline struct kvm_mmu_page *page_header(hpa_t shadow_page) >> { >> struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT); >> >> return (struct kvm_mmu_page *)(page->mapping); >> } >> >> which is used in the common code. > Ah, so the pointer is not available even after object is allocated. > Make sense since we allocate object, not page here, but is it safe to > use mapping like that? The commens says: struct address_space *mapping; /* If low bit clear, points to * inode address_space, or NULL. * If page mapped as anonymous * memory, low bit is set, and * it points to anon_vma object: * see PAGE_MAPPING_ANON below. It seems mapping is used for address_space or anonymous memory, in our case, the page is used by slab, so I guess it is ok. And the bug i put in set_page_header() was not tiggered on both slab and slub.