mm: pmd_read_atomic: fix 32bit PAE pmd walk vs pmd_populate SMP race condition

[android-x86/kernel.git] / include / asm-generic / pgtable.h

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 125c54e..6f2b45a 100644 (file)
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -158,9 +158,8 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_SPLITTING_FLUSH
-extern pmd_t pmdp_splitting_flush(struct vm_area_struct *vma,
-                                 unsigned long address,
-                                 pmd_t *pmdp);
+extern void pmdp_splitting_flush(struct vm_area_struct *vma,
+                                unsigned long address, pmd_t *pmdp);
 #endif
 
 #ifndef __HAVE_ARCH_PTE_SAME
@@ -446,6 +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
@@ -459,11 +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.