Memory allocations can happen before the swap_slots cache initialization
is completed during cpu bring up. If we are low on memory, we could call
get_swap_page and access swap_slots_cache before it is fully initialized.
Add a check in get_swap_page for initialized swap_slots_cache
to prevent this condition. Similar check already exists in
free_swap_slot. Also annotate the checks to indicate the likely
condition.
We also added a memory barrier to make sure that the locks
initialization are done before the assignment of cache->slots
and cache->slots_ret pointers. This ensures the assumption
that it is safe to acquire the slots cache locks and use the slots
cache when the corresponding cache->slots or cache->slots_ret
pointers are non null.
Reported-by: Wenwei Tao <wenwei.tww@xxxxxxxxxxxxxxx>
Acked-by: Ying Huang <ying.huang@xxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
mm/swap_slots.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 58f6c78..4c5457c 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -148,6 +148,14 @@ static int alloc_swap_slot_cache(unsigned int cpu)
cache->nr = 0;
cache->cur = 0;
cache->n_ret = 0;
+ /*
+ * We intialized alloc_lock and free_lock earlier.
+ * We use !cache->slots or !cache->slots_ret
+ * to know if it is safe to acquire the corresponding
+ * lock and use the cache. Memory barrier
+ * below ensures the assumption.
+ */
+ mb();
cache->slots = slots;
slots = NULL;
cache->slots_ret = slots_ret;
@@ -273,7 +281,7 @@ int free_swap_slot(swp_entry_t entry)
struct swap_slots_cache *cache;
cache = &get_cpu_var(swp_slots);
- if (use_swap_slot_cache && cache->slots_ret) {
+ if (likely(use_swap_slot_cache && cache->slots_ret)) {
spin_lock_irq(&cache->free_lock);
/* Swap slots cache may be deactivated before acquiring lock */
if (!use_swap_slot_cache) {
@@ -318,7 +326,7 @@ swp_entry_t get_swap_page(void)
cache = raw_cpu_ptr(&swp_slots);
entry.val = 0;
- if (check_cache_active()) {
+ if (likely(check_cache_active() && cache->slots)) {
mutex_lock(&cache->alloc_lock);
if (cache->slots) {
repeat:
--
2.9.4
--
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