Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Thu, 25 Jul 2019 21:21:22 +0800 |
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On 2019/7/25 下午4:28, Michael S. Tsirkin wrote: > On Thu, Jul 25, 2019 at 03:43:41PM +0800, Jason Wang wrote: >> On 2019/7/25 下午1:52, Michael S. Tsirkin wrote: >>> On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote: >>>> On 2019/7/23 下午5:26, Michael S. Tsirkin wrote: >>>>> 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. >>>> This is how I want to go with RFC and V1. But I end up with deadlock between >>>> vq locks and some MM internal locks. So I decide to use RCU which is 100% >>>> under the control of vhost. >>>> >>>> Thanks >>> And I guess the deadlock is because GUP is taking mmu locks which are >>> taken on mmu notifier path, right? >> >> Yes, but it's not the only lock. I don't remember the details, but I can >> confirm I meet issues with one or two other locks. >> >> >>> How about we add a seqlock and take >>> that in invalidate callbacks? We can then drop the VQ lock before GUP, >>> and take it again immediately after. >>> >>> something like >>> if (!vq_meta_mapped(vq)) { >>> vq_meta_setup(&uaddrs); >>> mutex_unlock(vq->mutex) >>> vq_meta_map(&uaddrs); >> >> The problem is the vq address could be changed at this time. >> >> >>> mutex_lock(vq->mutex) >>> >>> /* recheck both sock->private_data and seqlock count. */ >>> if changed - bail out >>> } >>> >>> And also requires that VQ uaddrs is defined like this: >>> - writers must have both vq mutex and dev mutex >>> - readers must have either vq mutex or dev mutex >>> >>> >>> That's a big change though. For now, how about switching to a per-vq SRCU? >>> That is only a little bit more expensive than RCU, and we >>> can use synchronize_srcu_expedited. >>> >> Consider we switch to use kfree_rcu(), what's the advantage of per-vq SRCU? >> >> Thanks > > I thought we established that notifiers must wait for > all readers to finish before they mark page dirty, to > prevent page from becoming dirty after address > has been invalidated. > Right?
Exactly, and that's the reason actually I use synchronize_rcu() there.
So the concern is still the possible synchronize_expedited()? Can I do this on through another series on top of the incoming V2?
Thanks
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