On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote: > > On 2019/3/8 上午3:16, Andrea Arcangeli wrote: > > On Thu, Mar 07, 2019 at 12:56:45PM -0500, Michael S. Tsirkin wrote: > > > On Thu, Mar 07, 2019 at 10:47:22AM -0500, Michael S. Tsirkin wrote: > > > > On Wed, Mar 06, 2019 at 02:18:12AM -0500, Jason Wang wrote: > > > > > +static const struct mmu_notifier_ops vhost_mmu_notifier_ops = { > > > > > + .invalidate_range = vhost_invalidate_range, > > > > > +}; > > > > > + > > > > > void vhost_dev_init(struct vhost_dev *dev, > > > > > struct vhost_virtqueue **vqs, int nvqs, int iov_limit) > > > > > { > > > > I also wonder here: when page is write protected then > > > > it does not look like .invalidate_range is invoked. > > > > > > > > E.g. mm/ksm.c calls > > > > > > > > mmu_notifier_invalidate_range_start and > > > > mmu_notifier_invalidate_range_end but not mmu_notifier_invalidate_range. > > > > > > > > Similarly, rmap in page_mkclean_one will not call > > > > mmu_notifier_invalidate_range. > > > > > > > > If I'm right vhost won't get notified when page is write-protected since you > > > > didn't install start/end notifiers. Note that end notifier can be called > > > > with page locked, so it's not as straight-forward as just adding a call. > > > > Writing into a write-protected page isn't a good idea. > > > > > > > > Note that documentation says: > > > > it is fine to delay the mmu_notifier_invalidate_range > > > > call to mmu_notifier_invalidate_range_end() outside the page table lock. > > > > implying it's called just later. > > > OK I missed the fact that _end actually calls > > > mmu_notifier_invalidate_range internally. So that part is fine but the > > > fact that you are trying to take page lock under VQ mutex and take same > > > mutex within notifier probably means it's broken for ksm and rmap at > > > least since these call invalidate with lock taken. > > Yes this lock inversion needs more thoughts. > > > > > And generally, Andrea told me offline one can not take mutex under > > > the notifier callback. I CC'd Andrea for why. > > Yes, the problem then is the ->invalidate_page is called then under PT > > lock so it cannot take mutex, you also cannot take the page_lock, it > > can at most take a spinlock or trylock_page. > > > > So it must switch back to the _start/_end methods unless you rewrite > > the locking. > > > > The difference with _start/_end, is that ->invalidate_range avoids the > > _start callback basically, but to avoid the _start callback safely, it > > has to be called in between the ptep_clear_flush and the set_pte_at > > whenever the pfn changes like during a COW. So it cannot be coalesced > > in a single TLB flush that invalidates all sptes in a range like we > > prefer for performance reasons for example in KVM. It also cannot > > sleep. > > > > In short ->invalidate_range must be really fast (it shouldn't require > > to send IPI to all other CPUs like KVM may require during an > > invalidate_range_start) and it must not sleep, in order to prefer it > > to _start/_end. > > > > I.e. the invalidate of the secondary MMU that walks the linux > > pagetables in hardware (in vhost case with GUP in software) has to > > happen while the linux pagetable is zero, otherwise a concurrent > > hardware pagetable lookup could re-instantiate a mapping to the old > > page in between the set_pte_at and the invalidate_range_end (which > > internally calls ->invalidate_range). Jerome documented it nicely in > > Documentation/vm/mmu_notifier.rst . > > > Right, I've actually gone through this several times but some details were > missed by me obviously. > > > > > > Now you don't really walk the pagetable in hardware in vhost, but if > > you use gup_fast after usemm() it's similar. > > > > For vhost the invalidate would be really fast, there are no IPI to > > deliver at all, the problem is just the mutex. > > > Yes. A possible solution is to introduce a valid flag for VA. Vhost may only > try to access kernel VA when it was valid. Invalidate_range_start() will > clear this under the protection of the vq mutex when it can block. Then > invalidate_range_end() then can clear this flag. An issue is blockable is > always false for range_end(). > Note that there can be multiple asynchronous concurrent invalidate_range callbacks. So a flag does not work but a counter of number of active invalidation would. See how KSM is doing it for instance in kvm_main.c The pattern for this kind of thing is: my_invalidate_range_start(start,end) { ... if (mystruct_overlap(mystruct, start, end)) { mystruct_lock(); mystruct->invalidate_count++; ... mystruct_unlock(); } } my_invalidate_range_end(start,end) { ... if (mystruct_overlap(mystruct, start, end)) { mystruct_lock(); mystruct->invalidate_count--; ... mystruct_unlock(); } } my_access_va(mystruct) { again: wait_on(!mystruct->invalidate_count) mystruct_lock(); if (mystruct->invalidate_count) { mystruct_unlock(); goto again; } GUP(); ... mystruct_unlock(); } Cheers, Jérôme