Unionfs: implement vm_ops->page_mkwrite

Some file systems (e.g., ext4) require it.  Reported by Ted Ts'o.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index 28b6ce8c0a94e946c3b66fed53e9a14c14ce547e..488a582d80e68e8b0f236b58e20654d54204c4ad 100644 (file)
--- a/fs/unionfs/mmap.c
+++ b/fs/unionfs/mmap.c
@@ -65,6 +65,41 @@ static int unionfs_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
        return err;
 }
 
+static int unionfs_page_mkwrite(struct vm_area_struct *vma,
+                               struct vm_fault *vmf)
+{
+       int err = 0;
+       struct file *file, *lower_file;
+       const struct vm_operations_struct *lower_vm_ops;
+       struct vm_area_struct lower_vma;
+
+       BUG_ON(!vma);
+       memcpy(&lower_vma, vma, sizeof(struct vm_area_struct));
+       file = lower_vma.vm_file;
+       lower_vm_ops = UNIONFS_F(file)->lower_vm_ops;
+       BUG_ON(!lower_vm_ops);
+       if (!lower_vm_ops->page_mkwrite)
+               goto out;
+
+       lower_file = unionfs_lower_file(file);
+       BUG_ON(!lower_file);
+       /*
+        * XXX: vm_ops->page_mkwrite may be called in parallel.
+        * Because we have to resort to temporarily changing the
+        * vma->vm_file to point to the lower file, a concurrent
+        * invocation of unionfs_page_mkwrite could see a different
+        * value.  In this workaround, we keep a different copy of the
+        * vma structure in our stack, so we never expose a different
+        * value of the vma->vm_file called to us, even temporarily.
+        * A better fix would be to change the calling semantics of
+        * ->page_mkwrite to take an explicit file pointer.
+        */
+       lower_vma.vm_file = lower_file;
+       err = lower_vm_ops->page_mkwrite(&lower_vma, vmf);
+out:
+       return err;
+}
+
 /*
  * XXX: the default address_space_ops for unionfs is empty.  We cannot set
  * our inode->i_mapping->a_ops to NULL because too many code paths expect
@@ -86,4 +121,5 @@ struct address_space_operations unionfs_dummy_aops = {
 
 struct vm_operations_struct unionfs_vm_ops = {
        .fault          = unionfs_fault,
+       .page_mkwrite   = unionfs_page_mkwrite,
 };