xen/balloon: ensure preemption is disabled when using a scratch page

In decrease_reservation(), if the kernel is preempted between updating
the mapping and updating the p2m then they may end up using different
scratch pages.

Use get_balloon_scratch_page() and put_balloon_scratch_page() which use
get_cpu_var() and put_cpu_var() to correctly disable preemption.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>

1 files changed, 7 insertions, 4 deletions

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 3101cf6daf56..4a43bc31ed3c 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -380,6 +380,7 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
         enum bp_state state = BP_DONE;
         unsigned long  pfn, i;
         struct page   *page;
+        struct page   *scratch_page;
         int ret;
         struct xen_memory_reservation reservation = {
                 .address_bits = 0,
@@ -399,6 +400,8 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
         if (nr_pages > ARRAY_SIZE(frame_list))
                 nr_pages = ARRAY_SIZE(frame_list);
 
+        scratch_page = get_balloon_scratch_page();
+
         for (i = 0; i < nr_pages; i++) {
                 page = alloc_page(gfp);
                 if (page == NULL) {
@@ -416,7 +419,7 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
                 if (xen_pv_domain() && !PageHighMem(page)) {
                         ret = HYPERVISOR_update_va_mapping(
                                 (unsigned long)__va(pfn << PAGE_SHIFT),
-                                pfn_pte(page_to_pfn(__get_cpu_var(balloon_scratch_page)),
+                                pfn_pte(page_to_pfn(scratch_page),
                                         PAGE_KERNEL_RO), 0);
                         BUG_ON(ret);
                 }
@@ -432,14 +435,14 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
                 pfn = mfn_to_pfn(frame_list[i]);
                 if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                         unsigned long p;
-                        struct page *pg;
-                        pg = __get_cpu_var(balloon_scratch_page);
-                        p = page_to_pfn(pg);
+                        p = page_to_pfn(scratch_page);
                         __set_phys_to_machine(pfn, pfn_to_mfn(p));
                 }
                 balloon_append(pfn_to_page(pfn));
         }
 
+        put_balloon_scratch_page();
+
         set_xen_guest_handle(reservation.extent_start, frame_list);
         reservation.nr_extents   = nr_pages;
         ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);