[PATCH 3.16 144/157] xen: let alloc_xenballooned_pages() fail if not enough memory free

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Juergen Gross <jgross@xxxxxxxx>

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@xxxxxxxx>
[bwh: Backported to 3.16: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/xen/balloon.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -502,8 +502,15 @@ static void balloon_process(struct work_
 				state = reserve_additional_memory(credit);
 		}
 
-		if (credit < 0)
-			state = decrease_reservation(-credit, GFP_BALLOON);
+		if (credit < 0) {
+			long n_pages;
+
+			n_pages = min(-credit, si_mem_available());
+			state = decrease_reservation(n_pages, GFP_BALLOON);
+			if (state == BP_DONE && n_pages != -credit &&
+			    n_pages < totalreserve_pages)
+				state = BP_EAGAIN;
+		}
 
 		state = update_schedule(state);
 
@@ -561,6 +568,9 @@ int alloc_xenballooned_pages(int nr_page
 			enum bp_state st;
 			if (page)
 				balloon_append(page);
+			if (si_mem_available() < nr_pages)
+				return -ENOMEM;
+
 			st = decrease_reservation(nr_pages - pgno,
 					highmem ? GFP_HIGHUSER : GFP_USER);
 			if (st != BP_DONE)
@@ -692,7 +702,7 @@ static int __init balloon_init(void)
 	balloon_stats.schedule_delay = 1;
 	balloon_stats.max_schedule_delay = 32;
 	balloon_stats.retry_count = 1;
-	balloon_stats.max_retry_count = RETRY_UNLIMITED;
+	balloon_stats.max_retry_count = 4;
 
 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
 	balloon_stats.hotplug_pages = 0;