Drivers: hv: balloon: Prevent the host from ballooning the guest too low

Based on the amount of memory being managed set a floor on how low the
guest can be ballooned.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 34 insertions, 1 deletions

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index f8fc99600de8..37873213e24f 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -523,6 +523,34 @@ static void process_info(struct hv_dynmem_device *dm, struct dm_info_msg *msg)
         }
 }
 
+unsigned long compute_balloon_floor(void)
+{
+        unsigned long min_pages;
+#define MB2PAGES(mb) ((mb) << (20 - PAGE_SHIFT))
+        /* Simple continuous piecewiese linear function:
+         *  max MiB -> min MiB  gradient
+         *       0         0
+         *      16        16
+         *      32        24
+         *     128        72    (1/2)
+         *     512       168    (1/4)
+         *    2048       360    (1/8)
+         *    8192       552    (1/32)
+         *   32768      1320
+         *  131072      4392
+         */
+        if (totalram_pages < MB2PAGES(128))
+                min_pages = MB2PAGES(8) + (totalram_pages >> 1);
+        else if (totalram_pages < MB2PAGES(512))
+                min_pages = MB2PAGES(40) + (totalram_pages >> 2);
+        else if (totalram_pages < MB2PAGES(2048))
+                min_pages = MB2PAGES(104) + (totalram_pages >> 3);
+        else
+                min_pages = MB2PAGES(296) + (totalram_pages >> 5);
+#undef MB2PAGES
+        return min_pages;
+}
+
 /*
  * Post our status as it relates memory pressure to the
  * host. Host expects the guests to post this status
@@ -552,9 +580,14 @@ static void post_status(struct hv_dynmem_device *dm)
          * The host expects the guest to report free memory.
          * Further, the host expects the pressure information to
          * include the ballooned out pages.
+         * For a given amount of memory that we are managing, we
+         * need to compute a floor below which we should not balloon.
+         * Compute this and add it to the pressure report.
          */
         status.num_avail = val.freeram;
-        status.num_committed = vm_memory_committed() + dm->num_pages_ballooned;
+        status.num_committed = vm_memory_committed() +
+                                dm->num_pages_ballooned +
+                                compute_balloon_floor();
 
         vmbus_sendpacket(dm->dev->channel, &status,
                                 sizeof(struct dm_status),