> 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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
