[RFC PATCH 0/3] um: clean up mm creation - another attempt

[Johannes Berg]

On Mon, 2023-09-25 at 15:27 +0100, Anton Ivanov wrote:
> On 25/09/2023 14:33, Johannes Berg wrote:
> > On Mon, 2023-09-25 at 14:29 +0100, Anton Ivanov wrote:
> > > I have rebased the preempt patch on top of these series.
> > > 
> > > PREEMPT works with some performance decrease.
> > > 
> > > VOLUNTARY deadlocks early in boot around the time it starts loading modules.
> > > 
> > > non-preemptible deadlocks very early in boot.
> > > 
> > 
> > Well I guess that means there's still some issue in here? Hmm.
> > 
> > Now I don't understand anything anymore, I guess.
> 
> PEBKAC. The tree got corrupted somewhere during rebase. Reapplying everything on top of a clean master fixed it.
> 
> So it all works.

OK, whew. At least now I no longer _completely_ doubt the mental model I
have of UML VM :-)

> With some performance penalties compared to the old approach, but works.

I still find this odd though, I don't see what the flush would possibly
do in a new (mm host) process that's not achievable in arch_dup_mmap()?

OK, so let's see - arch_dup_mmap() is _earlier_ than the fork_handler,
because that only happens on the very first switch into the process.
This is only when it gets scheduled. So we'd be looking for something
that copy_process() changes in the MM after copy_mm() and before it can
get scheduled?

I guess we could even move the flush it into copy_thread(), which is a
simpler patch too, but it felt a bit wrong, since that's about the
(guest!) process, not the mm.

But basically I don't see anything there - fork syscall tail-calls
kernel_clone(), which doesn't really do anything with the result of
copy_process() except wake it up, and copy_process() doesn't really do
anything either?

johannes