[PATCH 4.19 166/168] xen/balloon: fix accounting in alloc_xenballooned_pages error path

[Greg Kroah-Hartman]

From: Roger Pau Monne <roger.pau@xxxxxxxxxx>

commit 1951fa33ec259abdf3497bfee7b63e7ddbb1a394 upstream.

target_unpopulated is incremented with nr_pages at the start of the
function, but the call to free_xenballooned_pages will only subtract
pgno number of pages, and thus the rest need to be subtracted before
returning or else accounting will be skewed.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20200727091342.52325-2-roger.pau@xxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/xen/balloon.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -632,6 +632,12 @@ int alloc_xenballooned_pages(int nr_page
  out_undo:
 	mutex_unlock(&balloon_mutex);
 	free_xenballooned_pages(pgno, pages);
+	/*
+	 * NB: free_xenballooned_pages will only subtract pgno pages, but since
+	 * target_unpopulated is incremented with nr_pages at the start we need
+	 * to remove the remaining ones also, or accounting will be screwed.
+	 */
+	balloon_stats.target_unpopulated -= nr_pages - pgno;
 	return ret;
 }
 EXPORT_SYMBOL(alloc_xenballooned_pages);