Messages in this thread | | | Subject | Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address | From | Jason Wang <> | Date | Mon, 11 Mar 2019 15:18:46 +0800 |
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On 2019/3/8 下午10:58, Jerome Glisse wrote: > 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
Yes, this should work.
Thanks
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