Re: [PATCH] x86/p2m: fix xenmem_add_to_physmap_one double page removal

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Sep 14, 2021 at 04:32:53PM +0200, Jan Beulich wrote:
> On 14.09.2021 15:34, Roger Pau Monne wrote:
> > If the new gfn matches the previous one (ie: gfn == old_gpfn)
> 
> It took me a while to realize that you probably mean "gpfn" here.

Indeed. The variable names could probably do with some disambiguation.

> > xenmem_add_to_physmap_one will issue a duplicated call to
> > guest_physmap_remove_page with the same gfn, because the
> 
> Considering the local variable of this name, may I suggest to
> upper-case GFN in this case?
> 
> > get_gpfn_from_mfn call has been moved by commit f8582da041 to be
> > performed before the original page is removed. This leads to the
> > second guest_physmap_remove_page failing, which was not the case
> > before commit f8582da041.
> > 
> > Fix this by adding a check that prevents a second call to
> > guest_physmap_remove_page if the previous one has already removed the
> > backing page from that gfn.
> 
> Same here (if this remains; see below).
> 
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -2813,7 +2813,7 @@ int xenmem_add_to_physmap_one(
> >      }
> >  
> >      /* Unmap from old location, if any. */
> > -    if ( !rc && old_gpfn != INVALID_M2P_ENTRY )
> > +    if ( !rc && old_gpfn != INVALID_M2P_ENTRY && !gfn_eq(_gfn(old_gpfn), 
> > gpfn) )
> >          rc = guest_physmap_remove_page(d, _gfn(old_gpfn), mfn, 
> > PAGE_ORDER_4K);
> >  
> >      /* Map at new location. */
> > 
> 
> It feels unbalanced to suppress the remove, but keep the add in
> this case.

Well, there's a remove further up which is not skipped.

> Wouldn't we be better off skipping both, or short-
> circuiting the effective no-op even earlier? I recall considering
> to install a shortcut, but it didn't seem worth it. But now that
> you've found actual breakage, perhaps this need reconsidering.

Sure, just didn't think this corner case was worth adding a short
circuit. Let me send v2 with that approach if that's what you
prefer.

Thanks, Roger.