xen: let alloc_xenballooned_pages() fail if not enough memory free

commit a1078e821b605813b63bf6bca414a85f804d5c66 upstream.

Instead of trying to allocate pages with GFP_USER in
add_ballooned_pages() check the available free memory via
si_mem_available(). GFP_USER is far less limiting memory exhaustion
than the test via si_mem_available().

This will avoid dom0 running out of memory due to excessive foreign
page mappings especially on ARM and on x86 in PVH mode, as those don't
have a pre-ballooned area which can be used for foreign mappings.

As the normal ballooning suffers from the same problem don't balloon
down more than si_mem_available() pages in one iteration. At the same
time limit the default maximum number of retries.

This is part of XSA-300.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static void xen_online_page(struct page *page, unsigned int order)
{
	unsigned long i, size = (1 << order);
	unsigned long start_pfn = page_to_pfn(page);
	struct page *p;

	pr_debug("Online %lu pages starting at pfn 0x%lx\n", size, start_pfn);
	mutex_lock(&balloon_mutex);
	for (i = 0; i < size; i++) {
		p = pfn_to_page(start_pfn + i);
		__online_page_set_limits(p);
		__SetPageOffline(p);
		__balloon_append(p);
	}
	mutex_unlock(&balloon_mutex);
}

static void xen_online_page(struct page *page, unsigned int order)
{
	unsigned long i, size = (1 << order);
	unsigned long start_pfn = page_to_pfn(page);
	struct page *p;

	pr_debug("Online %lu pages starting at pfn 0x%lx\n", size, start_pfn);
	mutex_lock(&balloon_mutex);
	for (i = 0; i < size; i++) {
		p = pfn_to_page(start_pfn + i);
		__online_page_set_limits(p);
		__SetPageOffline(p);
		__balloon_append(p);
	}
	mutex_unlock(&balloon_mutex);
}
