Re: [PATCHv2 10/10] xen/balloon: pre-allocate p2m entries for ballooned pages

[Julien Grall <julien.grall@citrix.com>]

Hi David,

On 24/07/2015 12:47, David Vrabel wrote:
> Pages returned by alloc_xenballooned_pages() will be used for grant
> mapping which will call set_phys_to_machine() (in PV guests).
>
> Ballooned pages are set as INVALID_P2M_ENTRY in the p2m and thus may
> be using the (shared) missing tables and a subsequent
> set_phys_to_machine() will need to allocate new tables.
>
> Since the grant mapping may be done from a context that cannot sleep,
> the p2m entries must already be allocated.
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>   drivers/xen/balloon.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index fd6970f3..8932d10 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -541,6 +541,7 @@ int alloc_xenballooned_pages(int nr_pages, struct page **pages)
>   {
>   	int pgno = 0;
>   	struct page *page;
> +	int ret = -ENOMEM;
>
>   	mutex_lock(&balloon_mutex);
>
> @@ -550,6 +551,11 @@ int alloc_xenballooned_pages(int nr_pages, struct page **pages)
>   		page = balloon_retrieve(true);
>   		if (page) {
>   			pages[pgno++] = page;
> +#ifdef CONFIG_XEN_HAVE_PVMMU
> +			ret = xen_alloc_p2m_entry(page_to_pfn(page));

Don't you want to call this function only when the guest is not using 
auto-translated physmap?

> +			if (ret < 0)
> +				goto out_undo;
> +#endif
>   		} else {
>   			enum bp_state st;
>
> @@ -576,7 +582,7 @@ int alloc_xenballooned_pages(int nr_pages, struct page **pages)
>    out_undo:
>   	mutex_unlock(&balloon_mutex);
>   	free_xenballooned_pages(pgno, pages);
> -	return -ENOMEM;
> +	return ret;
>   }
>   EXPORT_SYMBOL(alloc_xenballooned_pages);
>
>

Regards,

-- 
Julien Grall