1 files changed, 45 insertions, 0 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index a4fd3680038b..a2e5ce1fa081 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -272,6 +272,51 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 }
 
 /*
+ * Some rmap walk that needs to find all ptes/hugepmds without false
+ * negatives (like migrate and split_huge_page) running concurrent
+ * with operations that copy or move pagetables (like mremap() and
+ * fork()) to be safe. They depend on the anon_vma "same_anon_vma"
+ * list to be in a certain order: the dst_vma must be placed after the
+ * src_vma in the list. This is always guaranteed by fork() but
+ * mremap() needs to call this function to enforce it in case the
+ * dst_vma isn't newly allocated and chained with the anon_vma_clone()
+ * function but just an extension of a pre-existing vma through
+ * vma_merge.
+ *
+ * NOTE: the same_anon_vma list can still be changed by other
+ * processes while mremap runs because mremap doesn't hold the
+ * anon_vma mutex to prevent modifications to the list while it
+ * runs. All we need to enforce is that the relative order of this
+ * process vmas isn't changing (we don't care about other vmas
+ * order). Each vma corresponds to an anon_vma_chain structure so
+ * there's no risk that other processes calling anon_vma_moveto_tail()
+ * and changing the same_anon_vma list under mremap() will screw with
+ * the relative order of this process vmas in the list, because we
+ * they can't alter the order of any vma that belongs to this
+ * process. And there can't be another anon_vma_moveto_tail() running
+ * concurrently with mremap() coming from this process because we hold
+ * the mmap_sem for the whole mremap(). fork() ordering dependency
+ * also shouldn't be affected because fork() only cares that the
+ * parent vmas are placed in the list before the child vmas and
+ * anon_vma_moveto_tail() won't reorder vmas from either the fork()
+ * parent or child.
+ */
+void anon_vma_moveto_tail(struct vm_area_struct *dst)
+{
+        struct anon_vma_chain *pavc;
+        struct anon_vma *root = NULL;
+
+        list_for_each_entry_reverse(pavc, &dst->anon_vma_chain, same_vma) {
+                struct anon_vma *anon_vma = pavc->anon_vma;
+                VM_BUG_ON(pavc->vma != dst);
+                root = lock_anon_vma_root(root, anon_vma);
+                list_del(&pavc->same_anon_vma);
+                list_add_tail(&pavc->same_anon_vma, &anon_vma->head);
+        }
+        unlock_anon_vma_root(root);
+}
+
+/*
  * Attach vma to its own anon_vma, as well as to the anon_vmas that
  * the corresponding VMA in the parent process is attached to.
  * Returns 0 on success, non-zero on failure.