Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 07, 2024 at 01:39:53PM -0800, Dave Hansen wrote:
> On 3/7/24 13:04, Yosry Ahmed wrote:
> > I thought about doing inc_mm_tlb_gen() when LAM is updated, but it felt
> > hacky and more importantly doesn't make it clear in switch_mm_irqs_off()
> > that we correctly handle LAM updates. We can certainly add a comment,
> > but I think an explicit check for CPU LAM vs. mm LAM is much clearer.
> > 
> > WDYT?
> 
> The mm generations are literally there so that if the mm changes that
> all the CPUs know they need an update.  Changing LAM enabling is 100%
> consistent with telling other CPUs that they need an update.
> 
> I'd be curious of Andy feels differently though.

The mm generations are TLB-specific and all the code using them implies
as such (e.g. look at the checks in switch_mm_irqs_off() when prev ==
next). We can go around and update comments and/or function names to
make them more generic, but this seems excessive. If we don't, the code
becomes less clear imo.

I agree that the use case here is essentially the same (let other
CPUs know they need to write CR3), but I still think that since the LAM
case is just a simple one-time enablement, an explicit check in
switch_mm_irqs_off() would be clearer.

Just my 2c, let me know what you prefer :)

> 
> >> Considering how fun this code path is, a little effort at an actual
> >> reproduction would be really appreciated.
> > 
> > I tried reproducing it but gave up quickly. We need a certain sequence
> > of events to happen:
> > 
> > CPU 1					CPU 2
> > kthread_use_mm()
> > 					/* user thread enables LAM */
> > 					context_switch()
> > context_switch() /* to user thread */
> 
> First, it would be fine to either create a new kthread for reproduction
> purposes or to hack an existing one.  For instance, have have the LAM
> prctl() take an extra ref on the mm and stick it in a global variable:
> 
> 	mmgrab(current->mm);
> 	global_mm = current->mm;
> 
> Then in the kthread, grab the mm and use it:
> 
> 	while (!global_mm);
> 	kthread_use_mm(global_mm);
> 	... check for the race
> 	mmdrop(global_mm);
> 
> You can also hackily wait for thread to move with a stupid spin loop:
> 
> 	while (smp_processor_id() != 1);
> 
> and then actually move it with sched_setaffinity() from userspace.  That
> can make it easier to get that series of events to happen in lockstep.

I will take a stab at doing something similar and let you know, thanks.