NFS: Don't SIGBUS if nfs_vm_page_mkwrite races with a cache invalidation

commit bc4866b upstream.

In the case where we lock the page, and then find out that the page has
been thrown out of the page cache, we should just return VM_FAULT_NOPAGE.
This is what block_page_mkwrite() does in these situations.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

fs/nfs/file.c

@@ -560,7 +560,7 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	struct file *filp = vma->vm_file;
 	struct dentry *dentry = filp->f_path.dentry;
 	unsigned pagelen;
-	int ret = -EINVAL;
+	int ret = VM_FAULT_NOPAGE;
 	struct address_space *mapping;
 
 	dfprintk(PAGECACHE, "NFS: vm_page_mkwrite(%s/%s(%ld), offset %lld)\n",
@@ -576,21 +576,20 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	if (mapping != dentry->d_inode->i_mapping)
 		goto out_unlock;
 
-	ret = 0;
 	pagelen = nfs_page_length(page);
 	if (pagelen == 0)
 		goto out_unlock;
 
-	ret = nfs_flush_incompatible(filp, page);
-	if (ret != 0)
-		goto out_unlock;
+	ret = VM_FAULT_LOCKED;
+	if (nfs_flush_incompatible(filp, page) == 0 &&
+	    nfs_updatepage(filp, page, 0, pagelen) == 0)
+		goto out;
 
-	ret = nfs_updatepage(filp, page, 0, pagelen);
+	ret = VM_FAULT_SIGBUS;
 out_unlock:
-	if (!ret)
-		return VM_FAULT_LOCKED;
 	unlock_page(page);
-	return VM_FAULT_SIGBUS;
+out:
+	return ret;
 }
 
 static const struct vm_operations_struct nfs_file_vm_ops = {