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

From: Jan Beulich <jbeulich@suse.com>
To: Paul Durrant <pdurrant@amazon.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"George Dunlap" <George.Dunlap@eu.citrix.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xenproject.org,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v6 3/4] mm: make MEMF_no_refcount pages safe to assign
Date: Wed, 29 Jan 2020 16:15:10 +0100	[thread overview]
Message-ID: <36b4b20b-60b5-806a-ca5d-8253823427ce@suse.com> (raw)
In-Reply-To: <20200129143831.1369-4-pdurrant@amazon.com>

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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