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

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Mon, 2011-03-07 at 18:06 +0000, 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

Since this is exported it should probably have "xen" somewhere in the
name.

A "get"/"put" naming scheme usually implies some sort of reference count
manipulation when used in the kernel. "alloc"/"free" might be better
here? Or maybe "take"/"return"? (I don't really like that one)

> + * @nr_pages: Number of pages to get
> + * @pages: pages returned
> + * @force: Try to balloon out more pages if needed

Is there any case where this isn't passed in as true?

> + * @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))
> +                             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);
> +             }
> +     }

Maybe we should kick the balloon worker thread here if current < target
or some such? e.g. to reverse the effect of a force==1 in the getter.

> +
> +     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);



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