Re: [Xen-devel] [PATCH 2/3] xen-balloon: Add interface to retrieve ballooned pages

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Mar 07, 2011 at 01:06:47PM -0500, Daniel De Graaf wrote:
> Pages that have been ballooned are useful for other Xen drivers doing
> grant table actions, because these pages have valid struct page/PFNs but
> have no valid MFN so are available for remapping.
> 
> Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> ---
>  drivers/xen/balloon.c |   54 
> +++++++++++++++++++++++++++++++++++++++++++++++++
>  include/xen/balloon.h |    3 ++
>  2 files changed, 57 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index b0a7a92..be53596 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -328,6 +328,60 @@ void balloon_set_new_target(unsigned long target)
>  }
>  EXPORT_SYMBOL_GPL(balloon_set_new_target);
>  
> +/**
> + * get_ballooned_pages - get pages that have been ballooned out
> + * @nr_pages: Number of pages to get
> + * @pages: pages returned
> + * @force: Try to balloon out more pages if needed
> + * @return number of pages retrieved
> + */
> +int get_ballooned_pages(int nr_pages, struct page** pages, int force)
> +{
> +     int rv = 0;
> +     struct page* page;
> +     mutex_lock(&balloon_mutex);
> +     /* Pages are pulled off the back of the queue to prefer highmem */
> +     while (rv < nr_pages) {
> +             if (list_empty(&ballooned_pages)) {
> +                     if (!force)
> +                             break;
> +                     if (decrease_reservation(nr_pages - rv))

I am looking at the implementation of decrease_reservation with
Daniel's Kipper patches ("xen/balloon: Protect against CPU exhaust by event/x 
process")
and his code will the functionality of trying to balloon out the amount of
pages you want. But if it can't balloon out all, it will try the best it
can. Meaning if nr_pages is 256 and rv is 0, and we only say get 100 of them,
this particular loop would keep on going forever.. until it got the rest
of those 156 pages which it might not.

Perhaps rebasing on top of Daniel's patch and utilize the BP_EAGAIN logic
to back-off, or just return -EBUSY with the amount returned?

> +                             force = 0;
> +             } else {
> +                     page = list_entry(ballooned_pages.prev,
> +                             struct page, lru);
> +                     list_del(&page->lru);
> +                     pages[rv++] = page;
> +             }
> +     }
> +     mutex_unlock(&balloon_mutex);
> +     return rv;
> +}
> +EXPORT_SYMBOL(get_ballooned_pages);
> +
> +/**
> + * put_ballooned_pages - return pages retrieved with get_ballooned_pages
> + * @nr_pages: Number of pages
> + * @pages: pages to return
> + */
> +void put_ballooned_pages(int nr_pages, struct page** pages)
> +{
> +     int i;
> +
> +     mutex_lock(&balloon_mutex);
> +
> +     for (i = 0; i < nr_pages; i++) {
> +             if (PageHighMem(pages[i])) {
> +                     list_add_tail(&pages[i]->lru, &ballooned_pages);
> +             } else {
> +                     list_add(&pages[i]->lru, &ballooned_pages);
> +             }
> +     }
> +
> +     mutex_unlock(&balloon_mutex);
> +}
> +EXPORT_SYMBOL(put_ballooned_pages);
> +
>  static int __init balloon_init(void)
>  {
>       unsigned long pfn, nr_pages, extra_pfn_end;
> diff --git a/include/xen/balloon.h b/include/xen/balloon.h
> index b2b7c21..5fc25fa 100644
> --- a/include/xen/balloon.h
> +++ b/include/xen/balloon.h
> @@ -19,3 +19,6 @@ struct balloon_stats {
>  extern struct balloon_stats balloon_stats;
>  
>  void balloon_set_new_target(unsigned long target);
> +
> +int get_ballooned_pages(int nr_pages, struct page** pages, int force);
> +void put_ballooned_pages(int nr_pages, struct page** pages);
> -- 
> 1.7.3.4
> 
> 
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