Re: [Xen-devel] [PATCH v6 3/4] mm: make MEMF_no_refcount pages safe to assign

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

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 29 January 2020 15:15
> 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>; Volodymyr Babchuk
> <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Subject: Re: [PATCH v6 3/4] mm: make MEMF_no_refcount pages safe to assign
> 
> On 29.01.2020 15:38, Paul Durrant wrote:
> > @@ -2371,6 +2383,8 @@ void free_domheap_pages(struct page_info *pg,
> unsigned int order)
> >
> >          if ( likely(d) && likely(d != dom_cow) )
> >          {
> > +            long pages = 0;
> > +
> >              /* NB. May recursively lock from relinquish_memory(). */
> >              spin_lock_recursive(&d->page_alloc_lock);
> >
> > @@ -2386,9 +2400,11 @@ void free_domheap_pages(struct page_info *pg,
> unsigned int order)
> >                      BUG();
> >                  }
> >                  arch_free_heap_page(d, &pg[i]);
> > +                if ( !(pg[i].count_info & PGC_no_refcount) )
> > +                    pages--;
> >              }
> >
> > -            drop_dom_ref = !domain_adjust_tot_pages(d, -(1 << order));
> > +            drop_dom_ref = !domain_adjust_tot_pages(d, pages);
> 
> Following from what I've just said on the previous patch, this needs
> further changing then as well. There'll need to be a per-domain
> "non-refcounted-pages" count, which - when transitioning from zero
> to non-zero is accompanied by obtaining a domain ref, and when
> transitioning back to zero causes this domain ref to be dropped.
> Otherwise, once the last ref-counted page was freed, the domain
> may become ready for final destruction, no matter how many non-
> refcounted pages there still are on its page lists. (An alternative
> model might be to include all pages in ->tot_pages, keep using just
> that for the domain ref acquire/release, and subtract the new
> count when e.g. comparing against ->max_pages.)

Yes, I think I'll adjust totpages unconditionally and then subtract the 
secondary count for comparison as it means I can leave the ref counting alone.

  Paul

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