Drivers: hv: hv_balloon: survive ballooning request with num_pages=0

... and simplify alloc_balloon_pages() interface by removing redundant
alloc_error from it.

If we happen to enter balloon_up() with balloon_wrk.num_pages = 0 we will enter
infinite 'while (!done)' loop as alloc_balloon_pages() will be always returning
0 and not setting alloc_error. We will also be sending a meaningless message to
the host on every iteration.

The 'alloc_unit == 1 && alloc_error -> num_ballooned == 0' change and
alloc_error elimination requires a special comment. We do alloc_balloon_pages()
with 2 different alloc_unit values and there are 4 different
alloc_balloon_pages() results, let's check them all.

alloc_unit = 512:
1) num_ballooned = 0, alloc_error = 0: we do 'alloc_unit=1' and retry pre- and
  post-patch.
2) num_ballooned > 0, alloc_error = 0: we check 'num_ballooned == num_pages'
  and act accordingly,  pre- and post-patch.
3) num_ballooned > 0, alloc_error > 0: we report this chunk and remain within
  the loop, no changes here.
4) num_ballooned = 0, alloc_error > 0: we do 'alloc_unit=1' and retry pre- and
  post-patch.

alloc_unit = 1:
1) num_ballooned = 0, alloc_error = 0: this can happen in two cases: when we
  passed 'num_pages=0' to alloc_balloon_pages() or when there was no space in
  bl_resp to place a single response. The second option is not possible as
  bl_resp is of PAGE_SIZE size and single response 'union dm_mem_page_range' is
  8 bytes, but the first one is (in theory, I think that Hyper-V host never
  places such requests). Pre-patch code loops forever, post-patch code sends
  a reply with more_pages = 0 and finishes.
2) num_ballooned > 0, alloc_error = 0: we ran out of space in bl_resp, we
  report partial success and remain within the loop, no changes pre- and
  post-patch.
3) num_ballooned > 0, alloc_error > 0: pre-patch code finishes, post-patch code
  does one more try and if there is no progress (we finish with
  'num_ballooned = 0') we finish. So we try a bit harder with this patch.
4) num_ballooned = 0, alloc_error > 0: both pre- and post-patch code enter
 'more_pages = 0' branch and finish.

So this patch has two real effects:
1) We reply with an empty response to 'num_pages=0' request.
2) We try a bit harder on alloc_unit=1 allocations (and reply with an empty
   tail reply in case we fail).

An empty reply should be supported by host as we were able to send it even with
pre-patch code when we were not able to allocate a single page.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 6 insertions, 13 deletions

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 4f5323cedab3..74312c88534a 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -1081,9 +1081,9 @@ static void free_balloon_pages(struct hv_dynmem_device *dm,
 
 
 
-static int  alloc_balloon_pages(struct hv_dynmem_device *dm, int num_pages,
-                         struct dm_balloon_response *bl_resp, int alloc_unit,
-                         bool *alloc_error)
+static int alloc_balloon_pages(struct hv_dynmem_device *dm, int num_pages,
+                               struct dm_balloon_response *bl_resp,
+                               int alloc_unit)
 {
         int i = 0;
         struct page *pg;
@@ -1104,11 +1104,8 @@ static int  alloc_balloon_pages(struct hv_dynmem_device *dm, int num_pages,
                                 __GFP_NOMEMALLOC | __GFP_NOWARN,
                                 get_order(alloc_unit << PAGE_SHIFT));
 
-                if (!pg) {
-                        *alloc_error = true;
+                if (!pg)
                         return i * alloc_unit;
-                }
-
 
                 dm->num_pages_ballooned += alloc_unit;
 
@@ -1140,7 +1137,6 @@ static void balloon_up(struct work_struct *dummy)
         struct dm_balloon_response *bl_resp;
         int alloc_unit;
         int ret;
-        bool alloc_error;
         bool done = false;
         int i;
         struct sysinfo val;
@@ -1173,18 +1169,15 @@ static void balloon_up(struct work_struct *dummy)
 
 
                 num_pages -= num_ballooned;
-                alloc_error = false;
                 num_ballooned = alloc_balloon_pages(&dm_device, num_pages,
-                                                bl_resp, alloc_unit,
-                                                 &alloc_error);
+                                                    bl_resp, alloc_unit);
 
                 if (alloc_unit != 1 && num_ballooned == 0) {
                         alloc_unit = 1;
                         continue;
                 }
 
-                if ((alloc_unit == 1 && alloc_error) ||
-                        (num_ballooned == num_pages)) {
+                if (num_ballooned == 0 || num_ballooned == num_pages) {
                         bl_resp->more_pages = 0;
                         done = true;
                         dm_device.state = DM_INITIALIZED;