On Wed, Jul 31, 2019 at 09:28:20PM +0800, Jason Wang wrote: > > On 2019/7/31 下午8:39, Jason Gunthorpe wrote: > > On Wed, Jul 31, 2019 at 04:46:53AM -0400, Jason Wang wrote: > > > We used to use RCU to synchronize MMU notifier with worker. This leads > > > calling synchronize_rcu() in invalidate_range_start(). But on a busy > > > system, there would be many factors that may slow down the > > > synchronize_rcu() which makes it unsuitable to be called in MMU > > > notifier. > > > > > > A solution is SRCU but its overhead is obvious with the expensive full > > > memory barrier. Another choice is to use seqlock, but it doesn't > > > provide a synchronization method between readers and writers. The last > > > choice is to use vq mutex, but it need to deal with the worst case > > > that MMU notifier must be blocked and wait for the finish of swap in. > > > > > > So this patch switches use a counter to track whether or not the map > > > was used. The counter was increased when vq try to start or finish > > > uses the map. This means, when it was even, we're sure there's no > > > readers and MMU notifier is synchronized. When it was odd, it means > > > there's a reader we need to wait it to be even again then we are > > > synchronized. > > You just described a seqlock. > > > Kind of, see my explanation below. > > > > > > We've been talking about providing this as some core service from mmu > > notifiers because nearly every use of this API needs it. > > > That would be very helpful. > > > > > > IMHO this gets the whole thing backwards, the common pattern is to > > protect the 'shadow pte' data with a seqlock (usually open coded), > > such that the mmu notififer side has the write side of that lock and > > the read side is consumed by the thread accessing or updating the SPTE. > > > Yes, I've considered something like that. But the problem is, mmu notifier > (writer) need to wait for the vhost worker to finish the read before it can > do things like setting dirty pages and unmapping page. It looks to me > seqlock doesn't provide things like this. The seqlock is usually used to prevent a 2nd thread from accessing the VA while it is being changed by the mm. ie you use something seqlocky instead of the ugly mmu_notifier_unregister/register cycle. You are supposed to use something simple like a spinlock or mutex inside the invalidate_range_start to serialized tear down of the SPTEs with their accessors. > write_seqcount_begin() > > map = vq->map[X] > > write or read through map->addr directly > > write_seqcount_end() > > > There's no rmb() in write_seqcount_begin(), so map could be read before > write_seqcount_begin(), but it looks to me now that this doesn't harm at > all, maybe we can try this way. That is because it is a write side lock, not a read lock. IIRC seqlocks have weaker barriers because the write side needs to be serialized in some other way. The requirement I see is you need invalidate_range_start to block until another thread exits its critical section (ie stops accessing the SPTEs). That is a spinlock/mutex. You just can't invent a faster spinlock by open coding something with barriers, it doesn't work. Jason
