Heiko Carstens wrote: >> What you've done with dup_mm() is probably the brute-force way that I >> would have done it had I just been trying to make a proof of concept or >> something. I'm worried that there are a bunch of corner cases that >> haven't been considered. >> >> What if someone else is poking around with ptrace or something similar >> and they bump the mm_users: >> >> + if (tsk->mm->context.pgstes) >> + return 0; >> + if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 || >> + tsk->mm != tsk->active_mm || tsk->mm->ioctx_list) >> + return -EINVAL; >> -------->HERE >> + tsk->mm->context.pgstes = 1; /* dirty little tricks .. */ >> + mm = dup_mm(tsk); >> >> It'll race, possibly fault in some other pages, and those faults will be >> lost during the dup_mm(). I think you need to be able to lock out all >> of the users of access_process_vm() before you go and do this. You also >> need to make sure that anyone who has looked at task->mm doesn't go and >> get a reference to it and get confused later when it isn't the task->mm >> any more. >> >> >>> Therefore, we need to reallocate the page table after fork() >>> once we know that task is going to be a hypervisor. That's what this >>> code does: reallocate a bigger page table to accomondate the extra >>> information. The task needs to be single-threaded when calling for >>> extended page tables. >>> >>> Btw: at fork() time, we cannot tell whether or not the user's going to >>> be a hypervisor. Therefore we cannot do this in fork. >>> >> Can you convert the page tables at a later time without doing a >> wholesale replacement of the mm? It should be a bit easier to keep >> people off the pagetables than keep their grubby mitts off the mm >> itself. >> > > Yes, as far as I can see you're right. And whatever we do in arch code, > after all it's just a work around to avoid a new clone flag. > If something like clone() with CLONE_KVM would be useful for more > architectures than just s390 then maybe we should try to get a flag. > > Oh... there are just two unused clone flag bits left. Looks like the > namespace changes ate up a lot of them lately. > > Well, we could still play dirty tricks like setting a bit in current > via whatever mechanism which indicates child-wants-extended-page-tables > and then just fork and be happy. > How about taking mmap_sem for write and converting all page tables in-place? I'd rather avoid the need to fork() when creating a VM. -- error compiling committee.c: too many arguments to function _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
