[Xen-devel] [PATCH 1/3] 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 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>
CC: alex@xxxxxxxxxxx
CC: dcrisan@xxxxxxxxxxxx
---
 drivers/xen/balloon.c |   10 ++++++++--
 include/xen/balloon.h |    1 +
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 930fb68..91bd599 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -88,6 +88,8 @@ 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)];
+struct page* balloon_trade_page;
+
 
 #ifdef CONFIG_HIGHMEM
 #define inc_totalhigh_pages() (totalhigh_pages++)
@@ -423,7 +425,7 @@ 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),
-                               __pte_ma(0), 0);
+                               pfn_pte(page_to_pfn(balloon_trade_page), 
PAGE_KERNEL_RO), 0);
                        BUG_ON(ret);
                }
 #endif
@@ -436,7 +438,7 @@ 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(balloon_trade_page)));
                balloon_append(pfn_to_page(pfn));
        }
 
@@ -591,6 +593,10 @@ static int __init balloon_init(void)
        if (!xen_domain())
                return -ENODEV;
 
+       balloon_trade_page = alloc_page(GFP_KERNEL);
+       if (balloon_trade_page == NULL)
+               return -ENOMEM;
+
        pr_info("xen/balloon: Initialising balloon driver.\n");
 
        balloon_stats.current_pages = xen_pv_domain()
diff --git a/include/xen/balloon.h b/include/xen/balloon.h
index cc2e1a7..5c01a15 100644
--- a/include/xen/balloon.h
+++ b/include/xen/balloon.h
@@ -22,6 +22,7 @@ struct balloon_stats {
 };
 
 extern struct balloon_stats balloon_stats;
+extern struct page *balloon_trade_page;
 
 void balloon_set_new_target(unsigned long target);
 
-- 
1.7.2.5


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