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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 <tim.c.chen@linux.intel.com> Reported-by: Wenwei Tao <wenwei.tww@alibaba-inc.com> Acked-by: Ying Huang <ying.huang@intel.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Rik van Riel <riel@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Hillf Danton <hdanton@sina.com> Cc: Hugh Dickins <hughd@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -149,6 +149,13 @@ static int alloc_swap_slot_cache(unsigned int cpu)
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cache->nr = 0;
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cache->cur = 0;
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cache->n_ret = 0;
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/*
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* We initialized alloc_lock and free_lock earlier. We use
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* !cache->slots or !cache->slots_ret to know if it is safe to acquire
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* the corresponding lock and use the cache. Memory barrier below
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* ensures the assumption.
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*/
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mb();
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cache->slots = slots;
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slots = NULL;
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cache->slots_ret = slots_ret;
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@ -275,7 +282,7 @@ int free_swap_slot(swp_entry_t entry)
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struct swap_slots_cache *cache;
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cache = raw_cpu_ptr(&swp_slots);
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if (use_swap_slot_cache && cache->slots_ret) {
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if (likely(use_swap_slot_cache && cache->slots_ret)) {
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spin_lock_irq(&cache->free_lock);
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/* Swap slots cache may be deactivated before acquiring lock */
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if (!use_swap_slot_cache || !cache->slots_ret) {
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@ -326,7 +333,7 @@ swp_entry_t get_swap_page(struct page *page)
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*/
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cache = raw_cpu_ptr(&swp_slots);
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if (check_cache_active()) {
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if (likely(check_cache_active() && cache->slots)) {
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mutex_lock(&cache->alloc_lock);
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if (cache->slots) {
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repeat:
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