Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 10 Aug 2015, Julien Grall wrote:
> Hi Stefano,
> 
> On 10/08/15 12:18, Stefano Stabellini wrote:
> >>                    /* Link back into the page tables if not highmem. */
> >> @@ -396,14 +413,15 @@ static enum bp_state increase_reservation(unsigned 
> >> long nr_pages)
> >>  static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t 
> >> gfp)
> >>  {
> >>    enum bp_state state = BP_DONE;
> >> -  unsigned long  pfn, i;
> >> +  unsigned long i;
> >>    struct page   *page;
> >>    int ret;
> >>    struct xen_memory_reservation reservation = {
> >>            .address_bits = 0,
> >> -          .extent_order = 0,
> >> +          .extent_order = EXTENT_ORDER,
> >>            .domid        = DOMID_SELF
> >>    };
> >> +  static struct page *pages[ARRAY_SIZE(frame_list)];
> > 
> > This array can be rather large: I would try to avoid it, see below.
> 
> [..]
> 
> > 
> > I would simply and avoid introducing a new array:
> >     pfn = (frame_list[i] << XEN_PAGE_SHIFT) >> PAGE_SHIFT;
> >     page = pfn_to_page(pfn);
> 
> Which won't work because the frame_list contains a gfn and not a pfn.
> We need to translate back the gfn into a pfn and the into a page.
> 
> The cost of the translation may be big and I wanted to avoid anymore
> XEN_PAGE_SHIFT in the code. In general we should avoid to deal with 4KB
> PFN when it's not necessary, it make the code more confusing to read.

That is true


> If your only concern is the size of the array, we could decrease the
> number of frames by batch. Or allocation the variable once a boot time.

Yes, that is my only concern. Allocating only nr_pages new struct page*
would be good enough I guess.

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