On Wed, 22 Aug 2012 18:29:55 +0200 Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > On Wed, Aug 22, 2012 at 02:03:41PM +0800, Xiao Guangrong wrote: > > On 08/21/2012 11:06 PM, Andrea Arcangeli wrote: > > > CPU0 CPU1 > > > oldpage[1] == 0 (both guest & host) > > > oldpage[0] = 1 > > > trigger do_wp_page > > > > We always do ptep_clear_flush before set_pte_at_notify(), > > at this point, we have done: > > pte = 0 and flush all tlbs > > > mmu_notifier_change_pte > > > spte = newpage + writable > > > guest does newpage[1] = 1 > > > vmexit > > > host read oldpage[1] == 0 > > > > It can not happen, at this point pte = 0, host can not > > access oldpage anymore, host read can generate #PF, it > > will be blocked on page table lock until CPU 0 release the lock. > > Agreed, this is why your fix is safe. > > ... > > Thanks a lot for fixing this subtle race! I'll take that as an ack. Unfortunately we weren't told the user-visible effects of the bug, which often makes it hard to determine which kernel versions should be patched. Please do always provide this information when fixing a bug. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>