On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote: > > On 2019/7/23 下午4:10, Michael S. Tsirkin wrote: > > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote: > > > On 2019/7/23 下午3:23, Michael S. Tsirkin wrote: > > > > > > Really let's just use kfree_rcu. It's way cleaner: fire and forget. > > > > > Looks not, you need rate limit the fire as you've figured out? > > > > See the discussion that followed. Basically no, it's good enough > > > > already and is only going to be better. > > > > > > > > > And in fact, > > > > > the synchronization is not even needed, does it help if I leave a comment to > > > > > explain? > > > > Let's try to figure it out in the mail first. I'm pretty sure the > > > > current logic is wrong. > > > > > > Here is what the code what to achieve: > > > > > > - The map was protected by RCU > > > > > > - Writers are: MMU notifier invalidation callbacks, file operations (ioctls > > > etc), meta_prefetch (datapath) > > > > > > - Readers are: memory accessor > > > > > > Writer are synchronized through mmu_lock. RCU is used to synchronized > > > between writers and readers. > > > > > > The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it > > > with readers (memory accessors) in the path of file operations. But in this > > > case, vq->mutex was already held, this means it has been serialized with > > > memory accessor. That's why I think it could be removed safely. > > > > > > Anything I miss here? > > > > > So invalidate callbacks need to reset the map, and they do > > not have vq mutex. How can they do this and free > > the map safely? They need synchronize_rcu or kfree_rcu right? > > > Invalidation callbacks need but file operations (e.g ioctl) not. > > > > > > And I worry somewhat that synchronize_rcu in an MMU notifier > > is a problem, MMU notifiers are supposed to be quick: > > > Looks not, since it can allow to be blocked and lots of driver depends on > this. (E.g mmu_notifier_range_blockable()). Right, they can block. So why don't we take a VQ mutex and be done with it then? No RCU tricks. > > > they are on a read side critical section of SRCU. > > > > If we could get rid of RCU that would be even better. > > > > But now I wonder: > > invalidate_start has to mark page as dirty > > (this is what my patch added, current code misses this). > > > Nope, current code did this but not the case when map need to be invalidated > in the vhost control path (ioctl etc). > > > > > > at that point kernel can come and make the page clean again. > > > > At that point VQ handlers can keep a copy of the map > > and change the page again. > > > We will increase invalidate_count which prevent the page being used by map. > > Thanks OK I think I got it, thanks! > > > > > > > At this point I don't understand how we can mark page dirty > > safely. > > > > > > > > > Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(), > > > > > > > (just a little bit more hard to trigger): > > > > > > AFAIK these never run in response to guest events. > > > > > > So they can take very long and guests still won't crash. > > > > > What if guest manages to escape to qemu? > > > > > > > > > > Thanks > > > > Then it's going to be slow. Why do we care? > > > > What we do not want is synchronize_rcu that guest is blocked on. > > > > > > > Ok, this looks like that I have some misunderstanding here of the reason why > > > synchronize_rcu() is not preferable in the path of ioctl. But in kvm case, > > > if rcu_expedited is set, it can triggers IPIs AFAIK. > > > > > > Thanks > > > > > Yes, expedited is not good for something guest can trigger. > > Let's just use kfree_rcu if we can. Paul said even though > > documentation still says it needs to be rate-limited, that > > part is basically stale and will get updated. > >
