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>

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index f8fc99600de8d99872eb8a41d16939b0f266d07f..37873213e24f5f55c6ccf136e5b7fe64cdcfeb3b 100644 (file)
--- 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),