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>
[bwh: Backported to 3.16: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 8db7abb6ef5abcdfd3dddba2cdb8f645ef289428..cdb0552077531773b30f49c27f430badc38fa5c8 100644 (file)
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -502,8 +502,15 @@ static void balloon_process(struct work_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_pages, struct page **pages, bool highmem)
                        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;