[Xen-devel] [PATCH] xen/balloon: don't alloc page while non-preemptible

[David Vrabel <david.vrabel@xxxxxxxxxx>]

From: David Vrabel <david.vrabel@xxxxxxxxxx>

get_balloon_scratch_page() disables preemption so we cannot call
alloc_page() in between get/put_balloon_scratch_page().  Shuffle bits
around in decrease_reservation() to avoid this.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 drivers/xen/balloon.c |   23 +++++++++++------------
 1 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index a50c6e3..b232908 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -398,8 +398,6 @@ 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) {
@@ -413,6 +411,12 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
 
                scrub_page(page);
 
+               /*
+                * Ballooned out frames are effectively replaced with
+                * a scratch frame.  Ensure direct mappings and the
+                * p2m are consistent.
+                */
+               scratch_page = get_balloon_scratch_page();
 #ifdef CONFIG_XEN_HAVE_PVMMU
                if (xen_pv_domain() && !PageHighMem(page)) {
                        ret = HYPERVISOR_update_va_mapping(
@@ -422,24 +426,19 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
                        BUG_ON(ret);
                }
 #endif
-       }
-
-       /* Ensure that ballooned highmem pages don't have kmaps. */
-       kmap_flush_unused();
-       flush_tlb_all();
-
-       /* No more mappings: invalidate P2M and add to balloon. */
-       for (i = 0; i < nr_pages; i++) {
-               pfn = mfn_to_pfn(frame_list[i]);
                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                        unsigned long p;
                        p = page_to_pfn(scratch_page);
                        __set_phys_to_machine(pfn, pfn_to_mfn(p));
                }
+               put_balloon_scratch_page();
+
                balloon_append(pfn_to_page(pfn));
        }
 
-       put_balloon_scratch_page();
+       /* Ensure that ballooned highmem pages don't have kmaps. */
+       kmap_flush_unused();
+       flush_tlb_all();
 
        set_xen_guest_handle(reservation.extent_start, frame_list);
        reservation.nr_extents   = nr_pages;
-- 
1.7.2.5


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