On 05/28/2012 05:05 PM, Xiao Guangrong wrote: >>> >>> >>> I think the reason we move refcount in current code is, we should increase the >>> refcount of the page we will mapped into shadow page table, since we always >>> decrease its refcount after it is mapped. (That is this patch does.) >>> >> >> >> As far as I can tell __get_user_pages_fast() will take the reference >> count in the page head in the first place. > > > IIUC, the refcount used in the Compound Page is like this: > > get_user_pages / get_page(page): > head_page = page->first_page; > if (page is not the head page) > page->__mapcount++ > head_page->_count++ > > > put_page(page): > head_page = page->first_page; > if (page is not the head page) > page->__mapcount-- > head_page->_count-- > Aha. The "right thing" we should be doing is running get_page() on every small page within the frame (we asked for a small page but are opportunistrically using the pages around it, without a proper ref). That's a bit slow though, so we cheat. Maybe we should do it anyway. Large page maps/unmaps should be rare. But I guess we can start with your fix. But what about shifting mask by one bit? Isn't it sufficient? - mask = KVM_PAGES_PER_HPAGE(level) - 1; + mask = KVM_PAGES_PER_HPAGE(level); + mask *= KVM_HOST_HPAGES_PER_HPAGE; + mask -= 1; This should move the reference to the right place. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
