files: Create fake name for remapped file if original name is busy

When live-migrating a container on NFS the link-remap cannot
create original file on target machine, since the file name
is still visible -- source CT is alive. The proper fix would
be to move file aside, then create link, then move original
file back, but since NFS changes file name anyway, we just
do the same -- generate another name for link.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -587,20 +587,50 @@ const struct fdtype_ops regfile_dump_ops = {
  * files, directories, fifos, etc.
  */
 
+static inline int rfi_remap(struct reg_file_info *rfi)
+{
+	return link(rfi->remap->path, rfi->path);
+}
+
 int open_path(struct file_desc *d,
 		int(*open_cb)(struct reg_file_info *, void *), void *arg)
 {
 	struct reg_file_info *rfi;
 	int tmp;
+	char *orig_path = NULL;
 
 	rfi = container_of(d, struct reg_file_info, d);
 
 	if (rfi->remap) {
 		mutex_lock(ghost_file_mutex);
-		if (link(rfi->remap->path, rfi->path) < 0) {
-			pr_perror("Can't link %s -> %s", rfi->path,
-					rfi->remap->path);
-			return -1;
+		if (rfi_remap(rfi) < 0) {
+			static char tmp_path[PATH_MAX];
+
+			if (errno != EEXIST) {
+				pr_perror("Can't link %s -> %s", rfi->path,
+						rfi->remap->path);
+				return -1;
+			}
+
+			/*
+			 * The file whose name we're trying to create
+			 * exists. Need to pick some other one, we're
+			 * going to remove it anyway.
+			 *
+			 * Strictly speaking, this is cheating, file
+			 * name shouldn't change. But since NFS with
+			 * its silly-rename doesn't care, why should we?
+			 */
+
+			orig_path = rfi->path;
+			rfi->path = tmp_path;
+			snprintf(tmp_path, sizeof(tmp_path), "%s.cr_link", orig_path);
+			pr_debug("Fake %s -> %s link\n", rfi->path, rfi->remap->path);
+
+			if (rfi_remap(rfi) < 0) {
+				pr_perror("Can't create even fake link!");
+				return -1;
+			}
 		}
 	}
 
@@ -617,6 +647,9 @@ int open_path(struct file_desc *d,
 			pr_info("Unlink the ghost %s\n", rfi->remap->path);
 			unlink(rfi->remap->path);
 		}
+
+		if (orig_path)
+			rfi->path = orig_path;
 		mutex_unlock(ghost_file_mutex);
 	}