1 files changed, 22 insertions, 7 deletions

diff --git a/mm/mlock.c b/mm/mlock.c
index d480cd6fc475..c59c420fd6e1 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -133,7 +133,10 @@ static void __munlock_isolation_failed(struct page *page)
 
 /**
  * munlock_vma_page - munlock a vma page
- * @page - page to be unlocked
+ * @page - page to be unlocked, either a normal page or THP page head
+ *
+ * returns the size of the page as a page mask (0 for normal page,
+ *         HPAGE_PMD_NR - 1 for THP head page)
  *
  * called from munlock()/munmap() path with page supposedly on the LRU.
  * When we munlock a page, because the vma where we found the page is being
@@ -148,21 +151,30 @@ static void __munlock_isolation_failed(struct page *page)
  */
 unsigned int munlock_vma_page(struct page *page)
 {
-        unsigned int page_mask = 0;
+        unsigned int nr_pages;
 
         BUG_ON(!PageLocked(page));
 
         if (TestClearPageMlocked(page)) {
-                unsigned int nr_pages = hpage_nr_pages(page);
+                nr_pages = hpage_nr_pages(page);
                 mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
-                page_mask = nr_pages - 1;
                 if (!isolate_lru_page(page))
                         __munlock_isolated_page(page);
                 else
                         __munlock_isolation_failed(page);
+        } else {
+                nr_pages = hpage_nr_pages(page);
         }
 
-        return page_mask;
+        /*
+         * Regardless of the original PageMlocked flag, we determine nr_pages
+         * after touching the flag. This leaves a possible race with a THP page
+         * split, such that a whole THP page was munlocked, but nr_pages == 1.
+         * Returning a smaller mask due to that is OK, the worst that can
+         * happen is subsequent useless scanning of the former tail pages.
+         * The NR_MLOCK accounting can however become broken.
+         */
+        return nr_pages - 1;
 }
 
 /**
@@ -440,7 +452,8 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 
         while (start < end) {
                 struct page *page = NULL;
-                unsigned int page_mask, page_increm;
+                unsigned int page_mask;
+                unsigned long page_increm;
                 struct pagevec pvec;
                 struct zone *zone;
                 int zoneid;
@@ -490,7 +503,9 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
                                 goto next;
                         }
                 }
-                page_increm = 1 + (~(start >> PAGE_SHIFT) & page_mask);
+                /* It's a bug to munlock in the middle of a THP page */
+                VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);
+                page_increm = 1 + page_mask;
                 start += page_increm * PAGE_SIZE;
 next:
                 cond_resched();