Re: [Xen-devel] [PATCH v6 2/4] mm: modify domain_adjust_tot_pages() to better handle a zero adjustment

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 29 January 2020 15:08
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxxxxx>;
> Ian Jackson <ian.jackson@xxxxxxxxxxxxx>; Julien Grall <julien@xxxxxxx>;
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>
> Subject: Re: [PATCH v6 2/4] mm: modify domain_adjust_tot_pages() to better
> handle a zero adjustment
> 
> On 29.01.2020 15:38, Paul Durrant wrote:
> > --- a/xen/common/memory.c
> > +++ b/xen/common/memory.c
> > @@ -727,8 +727,7 @@ static long
> memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
> >                               (j * (1UL << exch.out.extent_order)));
> >
> >                  spin_lock(&d->page_alloc_lock);
> > -                drop_dom_ref = (dec_count &&
> > -                                !domain_adjust_tot_pages(d, -
> dec_count));
> > +                drop_dom_ref = !domain_adjust_tot_pages(d, -dec_count);
> 
> And it's only now that I see it in this shape that it becomes
> clear to me why the change above shouldn't be done, and why in
> your other patch code should be written similar to the above:
> The abstract model requires that the domain reference be
> dropped only when ->tot_pages _transitions_ to zero. No drop
> should occur if the count was already zero. Granted this may
> be technically impossible in the specific case here, but the
> code would still better reflect this general model, to prevent
> it getting (mis-)cloned into other places.
> 

Ok, I guess I'll drop this and then make sure that free_domheap_pages() avoids 
an erroneous ref drop.

  Paul
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel