1 files changed, 20 insertions, 2 deletions

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index e2768f188f55..6f2b45a9b6bc 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -445,6 +445,18 @@ static inline int pmd_write(pmd_t pmd)
 #endif /* __HAVE_ARCH_PMD_WRITE */
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
+#ifndef pmd_read_atomic
+static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
+{
+        /*
+         * Depend on compiler for an atomic pmd read. NOTE: this is
+         * only going to work, if the pmdval_t isn't larger than
+         * an unsigned long.
+         */
+        return *pmdp;
+}
+#endif
+
 /*
  * This function is meant to be used by sites walking pagetables with
  * the mmap_sem hold in read mode to protect against MADV_DONTNEED and
@@ -458,11 +470,17 @@ static inline int pmd_write(pmd_t pmd)
  * undefined so behaving like if the pmd was none is safe (because it
  * can return none anyway). The compiler level barrier() is critically
  * important to compute the two checks atomically on the same pmdval.
+ *
+ * For 32bit kernels with a 64bit large pmd_t this automatically takes
+ * care of reading the pmd atomically to avoid SMP race conditions
+ * against pmd_populate() when the mmap_sem is hold for reading by the
+ * caller (a special atomic read not done by "gcc" as in the generic
+ * version above, is also needed when THP is disabled because the page
+ * fault can populate the pmd from under us).
  */
 static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
 {
-        /* depend on compiler for an atomic pmd read */
-        pmd_t pmdval = *pmd;
+        pmd_t pmdval = pmd_read_atomic(pmd);
         /*
          * The barrier will stabilize the pmdval in a register or on
          * the stack so that it will stop changing under the code.