On Mon, May 05, 2025 at 10:46:38PM +0200, Mark Kettenis wrote:
> > Date: Mon, 5 May 2025 17:25:02 +0200
> > From: Claudio Jeker <cjeker@diehard.n-r-g.com>
> > 
> > On Sun, May 04, 2025 at 08:38:06AM -0600, Theo de Raadt wrote:
> > > Martin Pieuchot <mpi@grenadille.net> wrote:
> > > 
> > > > > I do not understand any of the diff.  It seems to be trying to return a
> > > > > child to the parent, before resources are cleared.
> > > > 
> > > > It doesn't.  Order of operations didn't change.  The KERNEL_LOCK() is
> > > > there and asserted.
> > > 
> > > I did not mention the KERNEL_LOCK().
> > > 
> > > > I disagree.  I need to remove the reaper.  That *is* the problem.
> > > 
> > > You are using proc0 in an very dangerous way.
> > > 
> > > > > I've called this uvmspace_purge().  It would be a bit like
> > > > > 
> > > > >      uvm_unmap_remove(p->p_vmspace, 0, VM_MAXUSER_ADDRESS,
> > > > >          &dead_entries, FALSE, TRUE, TRUE)
> > > > > 
> > > > > Except there are clearly optimizations.  The function can sleep.  It is
> > > > > called quite early in process context.
> > > > > [...] 
> > > > 
> > > > There's no need to add a new function.  Calling uvm_exit() from exit1()
> > > > is enough and work but that's not buying us much.
> > > 
> > > As proc0?  Disagree strongly on this direction.  The process should free
> > > it's own pages out of it's own map.  It can do that if uvm_exit() is split.
> > > 
> > > But you've made up your mind to ignore everything everyone says.  You don't
> > > even reply to emails about the proposals.  Your public requests for feedback
> > > come off as fake.
> > 
> > Here is a minimal diff implementing uvm_purge() in the dumbest way
> > possible. It removes the reaper from the hot path without any scary hacks.
> > 
> > It only fixes uvm_map_teardown() so it can be called again.
> 
> But we shouldn't have to call it again no?

There is a complication that has to do with VM space sharing (which is
only used by proc0). So it is easier for now to offload proc0 handling to
uvm_exit() (which does the refcnt update and check if it hit 0).
 
I think we can kill VM space sharing and use real threads for kthread.
This would remove the vm space refcnt and make code a fair bit simpler.
guenther@ wanted that as well so maybe I look into that.

> > I tried to zap the pmap first (with pmap_remove of the full user address
> > space) but that is very slow.
> 
> Our page tables are typically only populated sparsely.  So this
> doesn't surprise me.

Yes, it was a try, I was hoping that pmap_remove would be walk the sparse
pmap tables efficently. It does not.
 
> > I think pmap can be optimized a lot here but we don't have an api
> > for that yet.
> 
> What optimizations are you thinking off?  The only "easy" optimization
> I found in the teardown was replacing the TLB flushes with a single
> ASID-based TLB flush on arm64.

So there are a few things I noticed. uvm_map_teardown() spends about the
same amount of time in uvm as in pmap. It calls pmap many many times so
there is a lot of call overhead.

The moment uvm purges the userland mappings a split KU arch (arm64,
sparc64, ...) can nuke the pmap, flush out the tlbs via ASID and then
instruct uvm_map_teardown() to skip any pmap calls.

For systems witk K + U in pmap (i386, amd64) the situation is more complex. 
In that case we can kill all U mappings, even do the ASID flush but we
need to keep the K mappings around and with that enough of pmap so that
cpu_switchto() is able to jump off the thread.
Still doing this will still allow uvm_map_teardown() to skip most pmap
calls.

As you said the pmap tables are populated very sparesly and uvm removes
these blocks in a inefficent way.
 
> Speaking about that optimization, I think your diff breaks it.  The
> optimization relies on calling pmap_deactivate() in cpu_exit() before
> teardown starts.  But now that you do the teardown in exit1() it
> happens before that call is made.  So that'll have to be fixed.

Agreed.
 
> > Index: kern/kern_exit.c
> > ===================================================================
> > RCS file: /cvs/src/sys/kern/kern_exit.c,v
> > diff -u -p -r1.245 kern_exit.c
> > --- kern/kern_exit.c	2 May 2025 05:04:38 -0000	1.245
> > +++ kern/kern_exit.c	5 May 2025 15:18:17 -0000
> > @@ -235,6 +235,10 @@ exit1(struct proc *p, int xexit, int xsi
> >  		free(pr->ps_libcpin.pn_pins, M_PINSYSCALL,
> >  		    pr->ps_libcpin.pn_npins * sizeof(u_int));
> >  
> > +		KERNEL_UNLOCK();
> > +		uvm_purge(pr);
> > +		KERNEL_LOCK();
> > +
> >  		/*
> >  		 * If parent has the SAS_NOCLDWAIT flag set, we're not
> >  		 * going to become a zombie.
> > Index: uvm/uvm_extern.h
> > ===================================================================
> > RCS file: /cvs/src/sys/uvm/uvm_extern.h,v
> > diff -u -p -r1.180 uvm_extern.h
> > --- uvm/uvm_extern.h	19 Nov 2024 06:18:26 -0000	1.180
> > +++ uvm/uvm_extern.h	5 May 2025 15:18:17 -0000
> > @@ -269,6 +269,7 @@ int			uvm_fault(vm_map_t, vaddr_t, vm_fa
> >  vaddr_t			uvm_uarea_alloc(void);
> >  void			uvm_uarea_free(struct proc *);
> >  void			uvm_exit(struct process *);
> > +void			uvm_purge(struct process *);
> >  void			uvm_init_limits(struct plimit *);
> >  boolean_t		uvm_kernacc(caddr_t, size_t, int);
> >  
> > Index: uvm/uvm_glue.c
> > ===================================================================
> > RCS file: /cvs/src/sys/uvm/uvm_glue.c,v
> > diff -u -p -r1.88 uvm_glue.c
> > --- uvm/uvm_glue.c	21 Mar 2025 13:19:33 -0000	1.88
> > +++ uvm/uvm_glue.c	5 May 2025 15:18:17 -0000
> > @@ -297,6 +297,29 @@ uvm_exit(struct process *pr)
> >  	uvmspace_free(vm);
> >  }
> >  
> > +void
> > +uvm_purge(struct process *pr)
> > +{
> > +	struct vmspace *vm = pr->ps_vmspace;
> > +
> > +	/* only proc0 uses shared vm space */
> > +	if (atomic_load_int(&vm->vm_refcnt) != 1)
> > +		return;
> > +
> > +#ifdef SYSVSHM
> > +	/* Get rid of any SYSV shared memory segments. */
> > +	if (vm->vm_shm != NULL) {
> > +		KERNEL_LOCK();
> > +		shmexit(vm);
> > +		KERNEL_UNLOCK();
> > +	}
> > +#endif
> > +
> > +//	pmap_remove(vm->vm_map.pmap, 0, VM_MAXUSER_ADDRESS);
> > +
> > +	uvm_map_teardown(&vm->vm_map);
> > +}
> > +
> >  /*
> >   * uvm_init_limit: init per-process VM limits
> >   *
> > Index: uvm/uvm_map.c
> > ===================================================================
> > RCS file: /cvs/src/sys/uvm/uvm_map.c,v
> > diff -u -p -r1.341 uvm_map.c
> > --- uvm/uvm_map.c	21 Apr 2025 14:46:18 -0000	1.341
> > +++ uvm/uvm_map.c	5 May 2025 15:18:17 -0000
> > @@ -138,7 +138,6 @@ int			 uvm_map_pageable_wire(struct vm_m
> >  			    vaddr_t, vaddr_t, int);
> >  void			 uvm_map_setup_entries(struct vm_map*);
> >  void			 uvm_map_setup_md(struct vm_map*);
> > -void			 uvm_map_teardown(struct vm_map*);
> >  void			 uvm_map_vmspace_update(struct vm_map*,
> >  			    struct uvm_map_deadq*, int);
> >  void			 uvm_map_kmem_grow(struct vm_map*,
> > @@ -2490,6 +2489,7 @@ uvm_map_teardown(struct vm_map *map)
> >  		/* Update wave-front. */
> >  		entry = TAILQ_NEXT(entry, dfree.deadq);
> >  	}
> > +	RBT_INIT(uvm_map_addr, &map->addr);
> >  
> >  	vm_map_unlock(map);
> >  
> > @@ -2512,9 +2512,6 @@ uvm_map_teardown(struct vm_map *map)
> >  	KASSERT(numt == numq);
> >  #endif
> >  	uvm_unmap_detach(&dead_entries, 0);
> > -
> > -	pmap_destroy(map->pmap);
> > -	map->pmap = NULL;
> >  }
> >  
> >  /*
> > @@ -3417,6 +3414,9 @@ uvmspace_free(struct vmspace *vm)
> >  #endif
> >  
> >  		uvm_map_teardown(&vm->vm_map);
> > +
> > +		pmap_destroy(vm->vm_map.pmap);
> > +
> >  		pool_put(&uvm_vmspace_pool, vm);
> >  	}
> >  }
> > Index: uvm/uvm_map.h
> > ===================================================================
> > RCS file: /cvs/src/sys/uvm/uvm_map.h,v
> > diff -u -p -r1.94 uvm_map.h
> > --- uvm/uvm_map.h	15 Nov 2024 02:59:23 -0000	1.94
> > +++ uvm/uvm_map.h	5 May 2025 15:18:17 -0000
> > @@ -359,6 +359,7 @@ void		uvm_unmap(struct vm_map *, vaddr_t
> >  void		uvm_unmap_detach(struct uvm_map_deadq *, int);
> >  int		uvm_unmap_remove(struct vm_map*, vaddr_t, vaddr_t,
> >  		    struct uvm_map_deadq *, boolean_t, boolean_t, boolean_t);
> > +void		uvm_map_teardown(struct vm_map*);
> >  void		uvm_map_set_uaddr(struct vm_map*, struct uvm_addr_state**,
> >  		    struct uvm_addr_state*);
> >  int		uvm_map_mquery(struct vm_map*, vaddr_t*, vsize_t, voff_t, int);
> > 
> > 

-- 
:wq Claudio