[PATCH] x86/pod: Do not fragment PoD memory allocations

[Elliott Mitchell <ehem+xen@m5p.com>]

Previously p2m_pod_set_cache_target() would fall back to allocating 4KB
pages if 2MB pages ran out.  This is counterproductive since it suggests
severe memory pressure and is likely a precursor to a memory exhaustion
panic.  As such don't try to fill requests for 2MB pages from 4KB pages
if 2MB pages run out.

Signed-off-by: Elliott Mitchell <ehem+xen@m5p.com>

---
I'm not including a separate cover message since this is a single hunk.
This really needs some checking in `xl`.  If one has a domain which
sometimes gets started on different hosts and is sometimes modified with
slightly differing settings, one can run into trouble.

In this case most of the time the particular domain is most often used
PV/PVH, but every so often is used as a template for HVM.  Starting it
HVM will trigger PoD mode.  If it is started on a machine with less
memory than others, PoD may well exhaust all memory and then trigger a
panic.

`xl` should likely fail HVM domain creation when the maximum memory
exceeds available memory (never mind total memory).
---
 xen/arch/x86/mm/p2m-pod.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index 48e609d1ed..6a7c9ae7d1 100644
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -216,12 +216,10 @@ p2m_pod_set_cache_target(struct p2m_domain *p2m, unsigned long pod_target, int p
         page = alloc_domheap_pages(d, order, 0);
         if ( unlikely(page == NULL) )
         {
-            if ( order == PAGE_ORDER_2M )
-            {
-                /* If we can't allocate a superpage, try singleton pages */
-                order = PAGE_ORDER_4K;
-                goto retry;
-            }
+            /* Superpages allocation failures likely indicate severe memory
+            ** pressure.  Continuing to try to fulfill attempts using 4KB pages
+            ** is likely to exhaust memory and trigger a panic.  As such it is
+            ** NOT worth trying to use 4KB pages to fulfill 2MB page requests.*/
 
             printk("%s: Unable to allocate page for PoD cache (target=%lu cache=%ld)\n",
                    __func__, pod_target, p2m->pod.count);
-- 
2.20.1