[PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Muchun Song]

There is a race condition between __free_huge_page()
and dissolve_free_huge_page().

CPU0:                         CPU1:

// page_count(page) == 1
put_page(page)
  __free_huge_page(page)
                              dissolve_free_huge_page(page)
                                spin_lock(&hugetlb_lock)
                                // PageHuge(page) && !page_count(page)
                                update_and_free_page(page)
                                // page is freed to the buddy
                                spin_unlock(&hugetlb_lock)
    spin_lock(&hugetlb_lock)
    clear_page_huge_active(page)
    enqueue_huge_page(page)
    // It is wrong, the page is already freed
    spin_unlock(&hugetlb_lock)

The race windows is between put_page() and spin_lock() which
is in the __free_huge_page().

We should make sure that the page is already on the free list
when it is dissolved.

Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle 
hugepage")
Signed-off-by: Muchun Song <songmuc...@bytedance.com>
---
 mm/hugetlb.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4741d60f8955..8ff138c17129 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -79,6 +79,21 @@ DEFINE_SPINLOCK(hugetlb_lock);
 static int num_fault_mutexes;
 struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
 
+static inline bool PageHugeFreed(struct page *head)
+{
+       return (unsigned long)head[3].mapping == -1U;
+}
+
+static inline void SetPageHugeFreed(struct page *head)
+{
+       head[3].mapping = (void *)-1U;
+}
+
+static inline void ClearPageHugeFreed(struct page *head)
+{
+       head[3].mapping = NULL;
+}
+
 /* Forward declaration */
 static int hugetlb_acct_memory(struct hstate *h, long delta);
 
@@ -1028,6 +1043,7 @@ static void enqueue_huge_page(struct hstate *h, struct 
page *page)
        list_move(&page->lru, &h->hugepage_freelists[nid]);
        h->free_huge_pages++;
        h->free_huge_pages_node[nid]++;
+       SetPageHugeFreed(page);
 }
 
 static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
@@ -1044,6 +1060,7 @@ static struct page *dequeue_huge_page_node_exact(struct 
hstate *h, int nid)
 
                list_move(&page->lru, &h->hugepage_activelist);
                set_page_refcounted(page);
+               ClearPageHugeFreed(page);
                h->free_huge_pages--;
                h->free_huge_pages_node[nid]--;
                return page;
@@ -1291,6 +1308,17 @@ static inline void destroy_compound_gigantic_page(struct 
page *page,
                                                unsigned int order) { }
 #endif
 
+/*
+ * Because we reuse the mapping field of some tail page structs, we should
+ * reset those mapping to initial value before @head is freed to the buddy
+ * allocator. The invalid value will be checked in the free_tail_pages_check().
+ */
+static inline void reset_tail_page_mapping(struct hstate *h, struct page *head)
+{
+       if (!hstate_is_gigantic(h))
+               head[3].mapping = TAIL_MAPPING;
+}
+
 static void update_and_free_page(struct hstate *h, struct page *page)
 {
        int i;
@@ -1298,6 +1326,7 @@ static void update_and_free_page(struct hstate *h, struct 
page *page)
        if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
                return;
 
+       reset_tail_page_mapping(h, page);
        h->nr_huge_pages--;
        h->nr_huge_pages_node[page_to_nid(page)]--;
        for (i = 0; i < pages_per_huge_page(h); i++) {
@@ -1504,6 +1533,7 @@ static void prep_new_huge_page(struct hstate *h, struct 
page *page, int nid)
        spin_lock(&hugetlb_lock);
        h->nr_huge_pages++;
        h->nr_huge_pages_node[nid]++;
+       ClearPageHugeFreed(page);
        spin_unlock(&hugetlb_lock);
 }
 
@@ -1770,6 +1800,14 @@ int dissolve_free_huge_page(struct page *page)
                int nid = page_to_nid(head);
                if (h->free_huge_pages - h->resv_huge_pages == 0)
                        goto out;
+
+               /*
+                * We should make sure that the page is already on the free list
+                * when it is dissolved.
+                */
+               if (unlikely(!PageHugeFreed(head)))
+                       goto out;
+
                /*
                 * Move PageHWPoison flag from head page to the raw error page,
                 * which makes any subpages rather than the error page reusable.
-- 
2.11.0