From: Mark Kettenis <mark.kettenis@xs4all.nl>
Subject: Re: km_alloc(9), UAREA and kv_pageable
To: Martin Pieuchot <mpi@grenadille.net>
Cc: tech@openbsd.org
Date: Sun, 20 Oct 2024 21:42:02 +0200

> Date: Sun, 20 Oct 2024 19:47:23 +0200
> From: Martin Pieuchot <mpi@grenadille.net>
> 
> On 20/10/24(Sun) 14:36, Mark Kettenis wrote:
> > > Date: Sun, 20 Oct 2024 11:37:57 +0200
> > > From: Martin Pieuchot <mpi@grenadille.net>
> > 
> > Sorry, I didn't have time yet to investigate.  But I'd rather not
> > change the km_alloc() API if it is not necessary, especially when the
> > it introduces a somewhat vague concept like "managed" that doesn't
> > even make sense in the context where it gets used.
> > 
> > > On 10/10/24(Thu) 10:40, Martin Pieuchot wrote:
> > > > On 09/10/24(Wed) 13:39, Mark Kettenis wrote:
> > > > > > Date: Wed, 9 Oct 2024 13:16:25 +0200
> > > > > > From: Martin Pieuchot <mpi@grenadille.net>
> > > > > > 
> > > > > > On 02/10/24(Wed) 17:28, Martin Pieuchot wrote:
> > > > > > > It is currently impossible to use km_alloc(9) to get a managed mapping
> > > > > > > backed with physical pages allocated up front.  I guess this is why
> > > > > > > uvm_km_kmemalloc_pla() is still used to allocate the UAREA.
> > > > > > > 
> > > > > > > To fix that, I suggest the diff below which turns the "kp_pageable" flag
> > > > > > > of km_alloc(9) into a "kp_managed".  This new flag no longer implies
> > > > > > > "kp_nomem" so I updated the description and fixed the remaining XXX to
> > > > > > > preserve existing behavior of the `kp_pageable' global.
> > > > > > > 
> > > > > > > With this change I believe we could also get rid of uvm_km_zalloc() used
> > > > > > > in the i386 pmap.
> > > > > > > 
> > > > > > > Comments?  Oks?
> > > > > > 
> > > > > > Anyone?
> > > > > 
> > > > > I don't really understand why this needs to be "managed" memory.  We
> > > > > stopped paging out the uarea ages ago, and I wonder if all this
> > > > > complexity is just from the time when we did.  Can't this just be
> > > > > kp_zero memory?
> > > > 
> > > > On amd64 using kp_zero results in the kernel faulting in loop from which
> > > > it cannot recover.  Could you please look at it and tell me if there is a
> > > > bug somewhere else?
> > 
> > Since the memory is no longer "managed", setguardpage() needs to use
> > pmap_kremove() instead of pmap_remove().
> 
> Nice found!
> 
> > The following diff works for me on both amd64 and arm64.
> 
> Lovely, should we test this on more architectures?  Or are you confident
> with it as it is?

Looks like amd64 is the only architecture that does that guardpage
thing.  So I think this is fine.

> > Index: arch/amd64/amd64/vm_machdep.c
> > ===================================================================
> > RCS file: /cvs/src/sys/arch/amd64/amd64/vm_machdep.c,v
> > diff -u -p -r1.47 vm_machdep.c
> > --- arch/amd64/amd64/vm_machdep.c	11 Apr 2023 00:45:07 -0000	1.47
> > +++ arch/amd64/amd64/vm_machdep.c	20 Oct 2024 12:34:07 -0000
> > @@ -135,8 +135,7 @@ cpu_exit(struct proc *p)
> >  void
> >  setguardpage(struct proc *p)
> >  {
> > -	pmap_remove(pmap_kernel(), (vaddr_t)p->p_addr + PAGE_SIZE,
> > -	    (vaddr_t)p->p_addr + 2 * PAGE_SIZE);
> > +	pmap_kremove((vaddr_t)p->p_addr + PAGE_SIZE, PAGE_SIZE);
> >  	pmap_update(pmap_kernel());
> >  }
> >  
> > Index: uvm/uvm_glue.c
> > ===================================================================
> > RCS file: /cvs/src/sys/uvm/uvm_glue.c,v
> > diff -u -p -r1.85 uvm_glue.c
> > --- uvm/uvm_glue.c	8 Oct 2024 02:29:10 -0000	1.85
> > +++ uvm/uvm_glue.c	20 Oct 2024 12:34:07 -0000
> > @@ -257,20 +257,18 @@ uvm_vsunlock_device(struct proc *p, void
> >  	uvm_km_free(kernel_map, kva, sz);
> >  }
> >  
> > +const struct kmem_va_mode kv_uarea = {
> > +	.kv_map = &kernel_map,
> > +	.kv_align = USPACE_ALIGN
> > +};
> > +
> >  /*
> >   * uvm_uarea_alloc: allocate the u-area for a new thread
> >   */
> >  vaddr_t
> >  uvm_uarea_alloc(void)
> >  {
> > -	vaddr_t uaddr;
> > -
> > -	uaddr = uvm_km_kmemalloc_pla(kernel_map, uvm.kernel_object, USPACE,
> > -	    USPACE_ALIGN, UVM_KMF_ZERO,
> > -	    no_constraint.ucr_low, no_constraint.ucr_high,
> > -	    0, 0, USPACE/PAGE_SIZE);
> > -
> > -	return (uaddr);
> > +	return (vaddr_t)km_alloc(USPACE, &kv_uarea, &kp_zero, &kd_waitok);
> >  }
> >  
> >  /*
> > @@ -282,7 +280,7 @@ uvm_uarea_alloc(void)
> >  void
> >  uvm_uarea_free(struct proc *p)
> >  {
> > -	uvm_km_free(kernel_map, (vaddr_t)p->p_addr, USPACE);
> > +	km_free(p->p_addr, USPACE, &kv_uarea, &kp_zero);
> >  	p->p_addr = NULL;
> >  }
> >  
>