xen/balloon: fix ballooned page accounting without hotplug enabled

BugLink: https://bugs.launchpad.net/bugs/1859640

[ Upstream commit c673ec61 ]

When CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is not defined
reserve_additional_memory() will set balloon_stats.target_pages to a
wrong value in case there are still some ballooned pages allocated via
alloc_xenballooned_pages().

This will result in balloon_process() no longer be triggered when
ballooned pages are freed in batches.
Reported-by: Nicholas Tsirakis <niko.tsirakis@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/xen/balloon.c

@@ -392,7 +392,8 @@ static struct notifier_block xen_memory_nb = {
 #else
 static enum bp_state reserve_additional_memory(void)
 {
-	balloon_stats.target_pages = balloon_stats.current_pages;
+	balloon_stats.target_pages = balloon_stats.current_pages +
+				     balloon_stats.target_unpopulated;
 	return BP_ECANCELED;
 }
 #endif /* CONFIG_XEN_BALLOON_MEMORY_HOTPLUG */