[Xen-devel] Re: [PATCH 2/3 v3] xen-balloon: Add interface to retrieve ballooned pages

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Tue, 2011-03-08 at 16:55 +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 |   61 ++++++++++++++++++++++++++++++++++++++++++++++--
>  include/xen/balloon.h |    3 ++
>  2 files changed, 61 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index b0a7a92..53deaec 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -116,14 +116,17 @@ static void balloon_append(struct page *page)
>  }
>  
>  /* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
> -static struct page *balloon_retrieve(void)
> +static struct page *balloon_retrieve(int prefer_highmem)

bool?

>  {
>       struct page *page;
>  
>       if (list_empty(&ballooned_pages))
>               return NULL;
>  
> -     page = list_entry(ballooned_pages.next, struct page, lru);
> +     if (prefer_highmem)
> +             page = list_entry(ballooned_pages.prev, struct page, lru);
> +     else
> +             page = list_entry(ballooned_pages.next, struct page, lru);
>       list_del(&page->lru);
>  
>       if (PageHighMem(page)) {
> @@ -198,7 +201,7 @@ static int increase_reservation(unsigned long nr_pages)
>               goto out;
>  
>       for (i = 0; i < rc; i++) {
> -             page = balloon_retrieve();
> +             page = balloon_retrieve(0);
>               BUG_ON(page == NULL);
>  
>               pfn = page_to_pfn(page);
> @@ -328,6 +331,58 @@ void balloon_set_new_target(unsigned long target)
>  }
>  EXPORT_SYMBOL_GPL(balloon_set_new_target);
>  
> +/**
> + * alloc_xenballooned_pages - get pages that have been ballooned out
> + * @nr_pages: Number of pages to get
> + * @pages: pages returned
> + * @return number of pages retrieved
> + */
> +int alloc_xenballooned_pages(int nr_pages, struct page** pages)
> +{
> +     int rv = 0;
> +     int alloc_failed = 0;
> +     struct page* page;
> +     mutex_lock(&balloon_mutex);
> +     while (rv < nr_pages) {
> +             page = balloon_retrieve(1);
> +             if (page) {
> +                     pages[rv++] = page;
> +             } else {
> +                     if (alloc_failed)
> +                             break;
> +                     if (decrease_reservation(nr_pages - rv))
> +                             alloc_failed = 1;

This loop and the "alloc_failed" thing seems pretty subtle.

If I'm reading correctly then if decrease_reservation didn't manage to
return the number of pages we asked for then we have a few more turns of
the handle to retrieve whatever we can, but we already know it's not
going to be everything we asked for, or decrease_reservation would have
succeeded).

The only caller of this is your 3/3 which just returns what it got and
fails if it didn't get everything it asked for.

At least until we have a second caller which requires otherwise (and I
don't think we will) I think this function should either return all or
nothing and should clean up after itself in the latter case, which saves
the caller a job too.

Ian.

> +             }
> +     }
> +     mutex_unlock(&balloon_mutex);
> +     return rv;
> +}
> +EXPORT_SYMBOL(alloc_xenballooned_pages);
> +
> +/**
> + * free_xenballooned_pages - return pages retrieved with get_ballooned_pages
> + * @nr_pages: Number of pages
> + * @pages: pages to return
> + */
> +void free_xenballooned_pages(int nr_pages, struct page** pages)
> +{
> +     int i;
> +
> +     mutex_lock(&balloon_mutex);
> +
> +     for (i = 0; i < nr_pages; i++) {
> +             if (pages[i])
> +                     balloon_append(pages[i]);
> +     }
> +
> +     /* The balloon may be too large now. Shrink it if needed. */
> +     if (current_target() != balloon_stats.current_pages)
> +             schedule_work(&balloon_worker);
> +
> +     mutex_unlock(&balloon_mutex);
> +}
> +EXPORT_SYMBOL(free_xenballooned_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..72a6927 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 alloc_xenballooned_pages(int nr_pages, struct page** pages);
> +void free_xenballooned_pages(int nr_pages, struct page** pages);



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