Messages in this thread | | | Subject | Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address | From | Jason Wang <> | Date | Fri, 8 Mar 2019 16:58:44 +0800 |
| |
On 2019/3/8 上午3:17, Jerome Glisse 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. >> >> And generally, Andrea told me offline one can not take mutex under >> the notifier callback. I CC'd Andrea for why. > Correct, you _can not_ take mutex or any sleeping lock from within the > invalidate_range callback as those callback happens under the page table > spinlock. You can however do so under the invalidate_range_start call- > back only if it is a blocking allow callback (there is a flag passdown > with the invalidate_range_start callback if you are not allow to block > then return EBUSY and the invalidation will be aborted). > > >> That's a separate issue from set_page_dirty when memory is file backed. > If you can access file back page then i suggest using set_page_dirty > from within a special version of vunmap() so that when you vunmap you > set the page dirty without taking page lock. It is safe to do so > always from within an mmu notifier callback if you had the page map > with write permission which means that the page had write permission > in the userspace pte too and thus it having dirty pte is expected > and calling set_page_dirty on the page is allowed without any lock. > Locking will happen once the userspace pte are tear down through the > page table lock.
Can I simply can set_page_dirty() before vunmap() in the mmu notifier callback, or is there any reason that it must be called within vumap()?
Thanks
> >> It's because of all these issues that I preferred just accessing >> userspace memory and handling faults. Unfortunately there does not >> appear to exist an API that whitelists a specific driver along the lines >> of "I checked this code for speculative info leaks, don't add barriers >> on data path please". > Maybe it would be better to explore adding such helper then remapping > page into kernel address space ? > > Cheers, > Jérôme
| |