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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan

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