rbd-nbd: fix unused nbd device search bug in container

In some container scenarios, the host may choose to
map a specific nbd device, for example, /dev/nbd6 into the
container, in that case, the nbd device available in the
container is not numbered from 0. The current unused
nbd device search function will return no result.
This patch fixes it.

Fixes: http://tracker.ceph.com/issues/22012
Signed-off-by: Li Wang <laurence.liwang@gmail.com>
Reviewed-by: Yunchuan Wen <yunchuan.wen@kylin-cloud.com>
(cherry picked from commit be0f9581f9727187ca03232e0b368e7da7a60609)

diff --git a/src/tools/rbd_nbd/rbd-nbd.cc b/src/tools/rbd_nbd/rbd-nbd.cc
index f9115a9e5dd312e1eb52a99a410887044fd3e820..ed28922d43f3f6a82f2c601c754ff971643993d1 100644 (file)
--- a/src/tools/rbd_nbd/rbd-nbd.cc
+++ b/src/tools/rbd_nbd/rbd-nbd.cc
@@ -558,12 +558,27 @@ static int do_map(int argc, const char *argv[])
   if (devpath.empty()) {
     char dev[64];
     bool try_load_module = true;
+    const char *path = "/sys/module/nbd/parameters/nbds_max";
+    int nbds_max = -1;
+    if (access(path, F_OK) == 0) {
+      std::ifstream ifs;
+      ifs.open(path, std::ifstream::in);
+      if (ifs.is_open()) {
+        ifs >> nbds_max;
+        ifs.close();
+      }
+    }
+
     while (true) {
       snprintf(dev, sizeof(dev), "/dev/nbd%d", index);
 
       nbd = open_device(dev, try_load_module);
       try_load_module = false;
       if (nbd < 0) {
+        if (nbd == -EPERM && nbds_max != -1 && index < (nbds_max-1)) {
+          ++index;
+          continue;
+        }
         r = nbd;
         cerr << "rbd-nbd: failed to find unused device" << std::endl;
         goto close_fd;