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Subject[PATCH RESEND 3/4] hv_balloon: fix bugs in num_pages_onlined accounting
Date
Our num_pages_onlined accounting is buggy:
1) In case we're offlining a memory block which was present at boot (e.g.
when there was no hotplug at all) we subtract 32k from 0 and as
num_pages_onlined is unsigned get a very big positive number.

2) Commit 6df8d9aaf3af ("Drivers: hv: balloon: Correctly update onlined
page count") made num_pages_onlined counter accurate on onlining but
totally incorrect on offlining for partly populated regions: no matter
how many pages were onlined and what was actually added to
num_pages_onlined counter we always subtract the full region (32k) so
again, num_pages_onlined can wrap around zero. By onlining/offlining
the same partly populated region multiple times we can make the
situation worse.

Solve these issues by doing accurate accounting on offlining: walk HAS
list, check for covered range and gaps.

Fixes: 6df8d9aaf3af ("Drivers: hv: balloon: Correctly update onlined page count")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
drivers/hv/hv_balloon.c | 82 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 69 insertions(+), 13 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 5b8e1ad1bcfe..7514a32d0de4 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -576,11 +576,65 @@ static struct hv_dynmem_device dm_device;
static void post_status(struct hv_dynmem_device *dm);

#ifdef CONFIG_MEMORY_HOTPLUG
+static inline bool has_pfn_is_backed(struct hv_hotadd_state *has,
+ unsigned long pfn)
+{
+ struct hv_hotadd_gap *gap;
+
+ /* The page is not backed. */
+ if ((pfn < has->covered_start_pfn) || (pfn >= has->covered_end_pfn))
+ return false;
+
+ /* Check for gaps. */
+ list_for_each_entry(gap, &has->gap_list, list) {
+ if ((pfn >= gap->start_pfn) && (pfn < gap->end_pfn))
+ return false;
+ }
+
+ return true;
+}
+
+static unsigned long hv_page_offline_check(unsigned long start_pfn,
+ unsigned long nr_pages)
+{
+ unsigned long pfn = start_pfn, count = 0;
+ struct hv_hotadd_state *has;
+ bool found;
+
+ while (pfn < start_pfn + nr_pages) {
+ /*
+ * Search for HAS which covers the pfn and when we find one
+ * count how many consequitive PFNs are covered.
+ */
+ found = false;
+ list_for_each_entry(has, &dm_device.ha_region_list, list) {
+ while ((pfn >= has->start_pfn) &&
+ (pfn < has->end_pfn) &&
+ (pfn < start_pfn + nr_pages)) {
+ found = true;
+ if (has_pfn_is_backed(has, pfn))
+ count++;
+ pfn++;
+ }
+ }
+
+ /*
+ * This PFN is not in any HAS (e.g. we're offlining a region
+ * which was present at boot), no need to account for it. Go
+ * to the next one.
+ */
+ if (!found)
+ pfn++;
+ }
+
+ return count;
+}
+
static int hv_memory_notifier(struct notifier_block *nb, unsigned long val,
void *v)
{
struct memory_notify *mem = (struct memory_notify *)v;
- unsigned long flags;
+ unsigned long flags, pfn_count;

switch (val) {
case MEM_ONLINE:
@@ -593,7 +647,19 @@ static int hv_memory_notifier(struct notifier_block *nb, unsigned long val,

case MEM_OFFLINE:
spin_lock_irqsave(&dm_device.ha_lock, flags);
- dm_device.num_pages_onlined -= mem->nr_pages;
+ pfn_count = hv_page_offline_check(mem->start_pfn,
+ mem->nr_pages);
+ if (pfn_count <= dm_device.num_pages_onlined) {
+ dm_device.num_pages_onlined -= pfn_count;
+ } else {
+ /*
+ * We're offlining more pages than we managed to online.
+ * This is unexpected. In any case don't let
+ * num_pages_onlined wrap around zero.
+ */
+ WARN_ON_ONCE(1);
+ dm_device.num_pages_onlined = 0;
+ }
spin_unlock_irqrestore(&dm_device.ha_lock, flags);
break;
case MEM_GOING_ONLINE:
@@ -612,19 +678,9 @@ static struct notifier_block hv_memory_nb = {
/* Check if the particular page is backed and can be onlined and online it. */
static void hv_page_online_one(struct hv_hotadd_state *has, struct page *pg)
{
- struct hv_hotadd_gap *gap;
- unsigned long pfn = page_to_pfn(pg);
-
- /* The page is not backed. */
- if ((pfn < has->covered_start_pfn) || (pfn >= has->covered_end_pfn))
+ if (!has_pfn_is_backed(has, page_to_pfn(pg)))
return;

- /* Check for gaps. */
- list_for_each_entry(gap, &has->gap_list, list) {
- if ((pfn >= gap->start_pfn) && (pfn < gap->end_pfn))
- return;
- }
-
/* This frame is currently backed; online the page. */
__online_page_set_limits(pg);
__online_page_increment_counters(pg);
--
2.14.3
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