[Xen-devel] [PATCH v4 1/2] xen/balloon: set a mapping for ballooned out pages

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Currently ballooned out pages are mapped to 0 and have INVALID_P2M_ENTRY
in the p2m. These ballooned out pages are used to map foreign grants
by gntdev and blkback (see alloc_xenballooned_pages).

Allocate a page per cpu and map all the ballooned out pages to the
corresponding mfn. Set the p2m accordingly. This way reading from a
ballooned out page won't cause a kernel crash (see
http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html).

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Reviewed-by: David Vrabel <david.vrabel@xxxxxxxxxx>
CC: alex@xxxxxxxxxxx
CC: dcrisan@xxxxxxxxxxxx
---
 drivers/xen/balloon.c |   69 ++++++++++++++++++++++++++++++++++++++++++++++--
 include/xen/balloon.h |    3 ++
 2 files changed, 69 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 930fb68..d5ff74f 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -36,6 +36,7 @@
  * IN THE SOFTWARE.
  */
 
+#include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/errno.h>
@@ -50,6 +51,7 @@
 #include <linux/notifier.h>
 #include <linux/memory.h>
 #include <linux/memory_hotplug.h>
+#include <linux/percpu-defs.h>
 
 #include <asm/page.h>
 #include <asm/pgalloc.h>
@@ -88,6 +90,8 @@ EXPORT_SYMBOL_GPL(balloon_stats);
 
 /* We increase/decrease in batches which fit in a page */
 static xen_pfn_t frame_list[PAGE_SIZE / sizeof(unsigned long)];
+static DEFINE_PER_CPU(struct page *, balloon_scratch_page);
+
 
 #ifdef CONFIG_HIGHMEM
 #define inc_totalhigh_pages() (totalhigh_pages++)
@@ -423,7 +427,8 @@ 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),
-                               __pte_ma(0), 0);
+                               
pfn_pte(page_to_pfn(__get_cpu_var(balloon_scratch_page)),
+                                       PAGE_KERNEL_RO), 0);
                        BUG_ON(ret);
                }
 #endif
@@ -436,7 +441,8 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
        /* No more mappings: invalidate P2M and add to balloon. */
        for (i = 0; i < nr_pages; i++) {
                pfn = mfn_to_pfn(frame_list[i]);
-               __set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
+               __set_phys_to_machine(pfn,
+                               
pfn_to_mfn(page_to_pfn(__get_cpu_var(balloon_scratch_page))));
                balloon_append(pfn_to_page(pfn));
        }
 
@@ -491,6 +497,18 @@ static void balloon_process(struct work_struct *work)
        mutex_unlock(&balloon_mutex);
 }
 
+struct page *get_balloon_scratch_page(void)
+{
+       struct page *ret = get_cpu_var(balloon_scratch_page);
+       BUG_ON(ret == NULL);
+       return ret;
+}
+
+void put_balloon_scratch_page(void)
+{
+       put_cpu_var(balloon_scratch_page);
+}
+
 /* Resets the Xen limit, sets new target, and kicks off processing. */
 void balloon_set_new_target(unsigned long target)
 {
@@ -584,13 +602,47 @@ static void __init balloon_add_region(unsigned long 
start_pfn,
        }
 }
 
+static int __cpuinit balloon_cpu_notify(struct notifier_block *self,
+                                   unsigned long action, void *hcpu)
+{
+       int cpu = (long)hcpu;
+       switch (action) {
+       case CPU_UP_PREPARE:
+               if (per_cpu(balloon_scratch_page, cpu) != NULL)
+                       break;
+               per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL);
+               if (per_cpu(balloon_scratch_page, cpu) == NULL) {
+                       pr_warn("Failed to allocate balloon_scratch_page for 
cpu %d\n", cpu);
+                       return NOTIFY_BAD;
+               }
+               break;
+       default:
+               break;
+       }
+       return NOTIFY_OK;
+}
+
+static struct notifier_block balloon_cpu_notifier __cpuinitdata = {
+       .notifier_call  = balloon_cpu_notify,
+};
+
 static int __init balloon_init(void)
 {
-       int i;
+       int i, cpu;
 
        if (!xen_domain())
                return -ENODEV;
 
+       for_each_online_cpu(cpu)
+       {
+               per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL);
+               if (per_cpu(balloon_scratch_page, cpu) == NULL) {
+                       pr_warn("Failed to allocate balloon_scratch_page for 
cpu %d\n", cpu);
+                       return -ENOMEM;
+               }
+       }
+       register_cpu_notifier(&balloon_cpu_notifier);
+
        pr_info("xen/balloon: Initialising balloon driver.\n");
 
        balloon_stats.current_pages = xen_pv_domain()
@@ -627,4 +679,15 @@ static int __init balloon_init(void)
 
 subsys_initcall(balloon_init);
 
+static int __init balloon_clear(void)
+{
+       int cpu;
+
+       for_each_possible_cpu(cpu)
+               per_cpu(balloon_scratch_page, cpu) = NULL;
+
+       return 0;
+}
+early_initcall(balloon_clear);
+
 MODULE_LICENSE("GPL");
diff --git a/include/xen/balloon.h b/include/xen/balloon.h
index cc2e1a7..a4c1c6a 100644
--- a/include/xen/balloon.h
+++ b/include/xen/balloon.h
@@ -29,6 +29,9 @@ int alloc_xenballooned_pages(int nr_pages, struct page 
**pages,
                bool highmem);
 void free_xenballooned_pages(int nr_pages, struct page **pages);
 
+struct page *get_balloon_scratch_page(void);
+void put_balloon_scratch_page(void);
+
 struct device;
 #ifdef CONFIG_XEN_SELFBALLOONING
 extern int register_xen_selfballooning(struct device *dev);
-- 
1.7.2.5


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