[CRIU] [PATCH] mount: treat non-openable mountpoint as unreachable

[Pavel Tikhomirov]

a) By default systemd creates a separate mount namespace for mysql,
setups "slave" mount propagation for /run and also mounts tmpfs to
/run/user in these ns, because of:

cat /usr/lib/systemd/system/mariadb.service | grep "/run/user" -A1
ProtectHome=true
rpm -qf /usr/lib/systemd/system/mariadb.service
MariaDB-server-10.2.10-1.el7.centos.x86_64

b) When ssh(root) is connected - systemd mounts tmpfs in container root
mount namespace to /run/user/0. As /run is slave mount /run/user/0 also
propagates to mysql's mount namespace and initially is put under
/run/user - becoming overmounted.

c) Criu is not yet able to dump overmounted tmpfs

So we have error when dumping mariadb CT with active ssh inside:

prlctl suspend mariadb-downloaded
Suspending the CT...
...
(02.432423) Error (criu/mount.c:1202): mnt: Can't open ./run/user/0: No
such file or directory
(02.465827) Error (criu/cr-dump.c:2005): Dumping FAILED.

To fix these we can just skip these mount_info in dump_one_fs and look
for a better bind-mount candidate, which, for our case, exists in
container root mntns.

note1: I'm not sure how to do a test for these as error depends on
the order of bindmounts in criu's representation.

note2: Other options is to support dumping overmounted mounts.

https://jira.sw.ru/browse/PSBM-57362
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 criu/mount.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/criu/mount.c b/criu/mount.c
index ccd3fef2c..d5cd7b535 100644
--- a/criu/mount.c
+++ b/criu/mount.c
@@ -1007,6 +1007,7 @@ int mnt_is_dir(struct mount_info *pm)
 	return 0;
 }
 
+#define MNT_UNREACHABLE INT_MIN
 /*
  * mnt_fd is a file descriptor on the mountpoint, which is closed in an error case.
  * If mnt_fd is -1, the mountpoint will be opened by this function.
@@ -1026,8 +1027,8 @@ int __open_mountpoint(struct mount_info *pm, int mnt_fd)
 
 		mnt_fd = openat(mntns_root, pm->ns_mountpoint, O_RDONLY);
 		if (mnt_fd < 0) {
-			pr_perror("Can't open %s", pm->ns_mountpoint);
-			return -1;
+			pr_warn("Can't open %s", pm->ns_mountpoint);
+			return MNT_UNREACHABLE;
 		}
 	}
 
@@ -1097,7 +1098,6 @@ static char *get_clean_mnt(struct mount_info *mi, char *mnt_path_tmp, char *mnt_
 	return mnt_path;
 }
 
-#define MNT_UNREACHABLE INT_MIN
 int open_mountpoint(struct mount_info *pm)
 {
 	struct mount_info *c;
-- 
2.13.6