From a2e16731728a285bcfcece0feaaa8cf478d24022 Mon Sep 17 00:00:00 2001 From: Tim Chen Date: Wed, 15 Nov 2017 17:34:18 -0800 Subject: [PATCH] mm/swap_slots.c: fix race conditions in swap_slots cache init 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. [akpm@linux-foundation.org: tidy up comment] [akpm@linux-foundation.org: fix spello in comment] Link: http://lkml.kernel.org/r/65a9d0f133f63e66bba37b53b2fd0464b7cae771.1500677066.git.tim.c.chen@linux.intel.com Signed-off-by: Tim Chen Reported-by: Wenwei Tao Acked-by: Ying Huang Cc: Minchan Kim Cc: Rik van Riel Cc: Andrea Arcangeli Cc: Johannes Weiner Cc: Michal Hocko Cc: Hillf Danton Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swap_slots.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/mm/swap_slots.c b/mm/swap_slots.c index d81cfc5a43d5..bebc19292018 100644 --- a/mm/swap_slots.c +++ b/mm/swap_slots.c @@ -149,6 +149,13 @@ static int alloc_swap_slot_cache(unsigned int cpu) cache->nr = 0; cache->cur = 0; cache->n_ret = 0; + /* + * We initialized 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; @@ -275,7 +282,7 @@ int free_swap_slot(swp_entry_t entry) struct swap_slots_cache *cache; cache = raw_cpu_ptr(&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 || !cache->slots_ret) { @@ -326,7 +333,7 @@ swp_entry_t get_swap_page(struct page *page) */ cache = raw_cpu_ptr(&swp_slots); - if (check_cache_active()) { + if (likely(check_cache_active() && cache->slots)) { mutex_lock(&cache->alloc_lock); if (cache->slots) { repeat: -- 2.34.1