libxl: Introduce a maximum limit for free_mem_slack (re (dom0) ballooning)

This fixes this message:
 libxl: error: libxl.c:2921:libxl_set_memory_target new target
               for dom0 is below the minimum threshold
which can occur spuriously if dom0_mem is specified and xl
autoballoning is left turned on.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 31535af788..8b086b9123 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2785,6 +2785,7 @@ static int libxl__fill_dom0_memory_info(libxl__gc *gc, uint32_t *target_memkb)
     char *free_mem_slack_path = "/local/domain/0/memory/freemem-slack";
     xs_transaction_t t;
     libxl_ctx *ctx = libxl__gc_owner(gc);
+    uint32_t free_mem_slack = 0;
 
 retry_transaction:
     t = xs_transaction_start(ctx->xsh);
@@ -2814,8 +2815,14 @@ retry_transaction:
             (uint32_t) info.current_memkb);
     libxl__xs_write(gc, t, max_path, "%"PRIu32,
             (uint32_t) info.max_memkb);
-    libxl__xs_write(gc, t, free_mem_slack_path, "%"PRIu32, (uint32_t)
-            (PAGE_TO_MEMKB(physinfo.total_pages) - info.current_memkb));
+
+    free_mem_slack = (uint32_t) (PAGE_TO_MEMKB(physinfo.total_pages) -
+            info.current_memkb);
+    /* From empirical measurements the free_mem_slack shouldn't be more
+     * than 15% of the total memory present on the system. */
+    if (free_mem_slack > PAGE_TO_MEMKB(physinfo.total_pages) * 0.15)
+        free_mem_slack = PAGE_TO_MEMKB(physinfo.total_pages) * 0.15;
+    libxl__xs_write(gc, t, free_mem_slack_path, "%"PRIu32, free_mem_slack);
 
     *target_memkb = (uint32_t) info.current_memkb;
     rc = 0;