Add new boot parameter 'lowmem_emergency_pool' which creates

an emergency pool of pages below 4GB (i.e., in Xen's DMA pool)
which will ever be used to satisfy normal allocation requests.

This is particularly useful in an i386 PAE environment to ensure
that pages will always be available for allocation as
page-table base directories.

Usage, for example (to reserve 16MB):
 lowmem_emergency_pool=16M

In a PAE environment, each reserved megabyte guarantees you should
be able to create up to 256 processes (counted across all guests).
16MB means you're good for 4096 processes.

Signed-off-by: Keir Fraser <keir@xensource.com>

1 files changed, 33 insertions, 1 deletions

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 4dff2f3679..763dafd75c 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -42,6 +42,20 @@
 static char opt_badpage[100] = "";
 string_param("badpage", opt_badpage);
 
+/*
+ * Amount of memory to reserve in a low-memory (<4GB) pool for specific
+ * allocation requests. Ordinary requests will not fall back to the
+ * lowmem emergency pool.
+ */
+static unsigned long lowmem_emergency_pool_pages;
+static void parse_lowmem_emergency_pool(char *s)
+{
+    unsigned long long bytes;
+    bytes = parse_size_and_unit(s);
+    lowmem_emergency_pool_pages = bytes >> PAGE_SHIFT;
+}
+custom_param("lowmem_emergency_pool", parse_lowmem_emergency_pool);
+
 #define round_pgdown(_p)  ((_p)&PAGE_MASK)
 #define round_pgup(_p)    (((_p)+(PAGE_SIZE-1))&PAGE_MASK)
 
@@ -514,7 +528,15 @@ struct page_info *alloc_domheap_pages(
     ASSERT(!in_irq());
 
     if ( !(flags & ALLOC_DOM_DMA) )
+    {
         pg = alloc_heap_pages(MEMZONE_DOM, order);
+        /* Failure? Then check if we can fall back to the DMA pool. */
+        if ( unlikely(pg == NULL) &&
+             ((order > MAX_ORDER) ||
+              (avail[MEMZONE_DMADOM] <
+               (lowmem_emergency_pool_pages + (1UL << order)))) )
+            return NULL;
+    }
 
     if ( pg == NULL )
         if ( (pg = alloc_heap_pages(MEMZONE_DMADOM, order)) == NULL )
@@ -657,7 +679,17 @@ void free_domheap_pages(struct page_info *pg, unsigned int order)
 
 unsigned long avail_domheap_pages(void)
 {
-    return avail[MEMZONE_DOM] + avail[MEMZONE_DMADOM];
+    unsigned long avail_nrm, avail_dma;
+
+    avail_nrm = avail[MEMZONE_DOM];
+
+    avail_dma = avail[MEMZONE_DMADOM];
+    if ( avail_dma > lowmem_emergency_pool_pages )
+        avail_dma -= lowmem_emergency_pool_pages;
+    else
+        avail_dma = 0;
+
+    return avail_nrm + avail_dma;
 }