Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

From: Jason Wang
Date: Mon Aug 05 2019 - 00:33:58 EST


On 2019/8/2 äå10:03, Michael S. Tsirkin wrote:
On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
Btw, I come up another idea, that is to disable preemption when vhost thread
need to access the memory. Then register preempt notifier and if vhost
thread is preempted, we're sure no one will access the memory and can do the
cleanup.
Great, more notifiers :(

Maybe can live with
1- disable preemption while using the cached pointer
2- teach vhost to recover from memory access failures,
by switching to regular from/to user path


I don't get this, I believe we want to recover from regular from/to user path, isn't it?



So if you want to try that, fine since it's a step in
the right direction.

But I think fundamentally it's not what we want to do long term.


Yes.



It's always been a fundamental problem with this patch series that only
metadata is accessed through a direct pointer.

The difference in ways you handle metadata and data is what is
now coming and messing everything up.


I do propose soemthing like this in the past: https://www.spinics.net/lists/linux-virtualization/msg36824.html. But looks like you have some concern about its locality.

But the problem still there, GUP can do page fault, so still need to synchronize it with MMU notifiers. The solution might be something like moving GUP to a dedicated kind of vhost work.



So if continuing the direct map approach,
what is needed is a cache of mapped VM memory, then on a cache miss
we'd queue work along the lines of 1-2 above.

That's one direction to take. Another one is to give up on that and
write our own version of uaccess macros. Add a "high security" flag to
the vhost module and if not active use these for userspace memory
access.


Or using SET_BACKEND_FEATURES? But do you mean permanent GUP as I did in original RFC https://lkml.org/lkml/2018/12/13/218?

Thanks