Messages in this thread | | | Date | Thu, 24 Oct 2013 15:32:39 +0300 | From | Gleb Natapov <> | Subject | Re: [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab |
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On Thu, Oct 24, 2013 at 07:01:49PM +0800, Xiao Guangrong wrote: > 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. > Yeah, I also think so. I asked Andrea (copied) and he thinks that it is safe too currently, but things changes fast in this area. Andrea? Another option is too save slab_cache pointer and reset it before freeing the object but it looks ugly.
-- Gleb.
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