[PATCH 3/6] Drivers: hv: hv_balloon: don't lose memory when onlining order is not natural

From: K. Y. Srinivasan
Date: Wed Mar 11 2015 - 20:41:15 EST

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Memory blocks can be onlined in random order. When this order is not natural
some memory pages are not onlined because of the redundant check in

Here is a real world scenario:
1) Host tries to hot-add the following (process_hot_add):
pg_start=rg_start=0x48000, pfn_cnt=111616, rg_size=262144

2) This results in adding 4 memory blocks:
[ 109.057866] init_memory_mapping: [mem 0x48000000-0x4fffffff]
[ 114.102698] init_memory_mapping: [mem 0x50000000-0x57ffffff]
[ 119.168039] init_memory_mapping: [mem 0x58000000-0x5fffffff]
[ 124.233053] init_memory_mapping: [mem 0x60000000-0x67ffffff]
The last one is incomplete but we have special has->covered_end_pfn counter to
avoid onlining non-backed frames and hv_bring_pgs_online() function to bring
them online later on.

3) Now we have 4 offline memory blocks: /sys/devices/system/memory/memory9-12
$ for f in /sys/devices/system/memory/memory*/state; do echo $f `cat $f`; done | grep -v onlin
/sys/devices/system/memory/memory10/state offline
/sys/devices/system/memory/memory11/state offline
/sys/devices/system/memory/memory12/state offline
/sys/devices/system/memory/memory9/state offline

4) We bring them online in non-natural order:
$grep MemTotal /proc/meminfo
MemTotal: 966348 kB
$echo online > /sys/devices/system/memory/memory12/state && grep MemTotal /proc/meminfo
MemTotal: 1019596 kB
$echo online > /sys/devices/system/memory/memory11/state && grep MemTotal /proc/meminfo
MemTotal: 1150668 kB
$echo online > /sys/devices/system/memory/memory9/state && grep MemTotal /proc/meminfo
MemTotal: 1150668 kB

As you can see memory9 block gives us zero additional memory. We can also
observe a huge discrepancy between host- and guest-reported memory sizes.

The root cause of the issue is the redundant pg >= covered_start_pfn check (and
covered_start_pfn advancing) in hv_online_page(). When upper memory block in
being onlined before the lower one (memory12 and memory11 in the above case) we
advance the covered_start_pfn pointer and all memory9 pages do not pass the
check. If the assumption that host always gives us requests in sequential order
and pg_start always equals rg_start when the first request for the new HA
region is received (that's the case in my testing) is correct than we can get
rid of covered_start_pfn and pg >= start_pfn check in hv_online_page() is

The current Linux 4.0 RC3 tree is broken and this patch fixes the problem.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
drivers/hv/hv_balloon.c | 14 ++++----------
1 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index f1f17c5..014256a 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -428,14 +428,13 @@ struct dm_info_msg {
* currently hot added. We hot add in multiples of 128M
* chunks; it is possible that we may not be able to bring
* online all the pages in the region. The range
- * covered_start_pfn : covered_end_pfn defines the pages that can
+ * covered_end_pfn defines the pages that can
* be brough online.

struct hv_hotadd_state {
struct list_head list;
unsigned long start_pfn;
- unsigned long covered_start_pfn;
unsigned long covered_end_pfn;
unsigned long ha_end_pfn;
unsigned long end_pfn;
@@ -679,8 +678,7 @@ static void hv_online_page(struct page *pg)

list_for_each(cur, &dm_device.ha_region_list) {
has = list_entry(cur, struct hv_hotadd_state, list);
- cur_start_pgp = (unsigned long)
- pfn_to_page(has->covered_start_pfn);
+ cur_start_pgp = (unsigned long)pfn_to_page(has->start_pfn);
cur_end_pgp = (unsigned long)pfn_to_page(has->covered_end_pfn);

if (((unsigned long)pg >= cur_start_pgp) &&
@@ -692,7 +690,6 @@ static void hv_online_page(struct page *pg)
- has->covered_start_pfn++;
@@ -736,10 +733,9 @@ static bool pfn_covered(unsigned long start_pfn, unsigned long pfn_cnt)
* is, update it.

- if (has->covered_end_pfn != start_pfn) {
+ if (has->covered_end_pfn != start_pfn)
has->covered_end_pfn = start_pfn;
- has->covered_start_pfn = start_pfn;
- }
return true;

@@ -784,7 +780,6 @@ static unsigned long handle_pg_range(unsigned long pg_start,
pgs_ol = pfn_cnt;
hv_bring_pgs_online(start_pfn, pgs_ol);
has->covered_end_pfn += pgs_ol;
- has->covered_start_pfn += pgs_ol;
pfn_cnt -= pgs_ol;

@@ -845,7 +840,6 @@ static unsigned long process_hot_add(unsigned long pg_start,
list_add_tail(&ha_region->list, &dm_device.ha_region_list);
ha_region->start_pfn = rg_start;
ha_region->ha_end_pfn = rg_start;
- ha_region->covered_start_pfn = pg_start;
ha_region->covered_end_pfn = pg_start;
ha_region->end_pfn = rg_start + rg_size;

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