Jffs2 vs ext4 overlay, strange behaviour

For Developers
1

I am trying to run greengrass core application on OpenWRT.
Daemon starts. but any attempt to deploy a function to it ends with panic state and application kills all its workers.
This is how the application behaves when /overlay is mounted on mtd partition (default system's configuration). But when I put /overlay on extroot (pendrive with ext4) the application works.

It seems that file permissions, attributes and owners are the same on both partitions.
Sometimes there is an error from sqlite:

SQLite error: Failed to Exec PRAGMA journal_mode: disk I/O error

and it is the only error message with some useful information.
Any idea what is going on here? How to debug it?

Tested on LEDE 17.01 and OpenWRT 18.06.

2 
opkg update; opkg install strace
strace -e trace=file -o /tmp/strace.log YOURPROGRAM
cat /tmp/strace.log | more
3

I don't see anything helpful in log, but I examined the disk I/O error.

Does JFFS2 support MMAP_SHARED? What about UBIFS

4

Is there enough free space on the device?

5

Yes it is. I've got ~1GB free.

But it must be mmap:

SQLite version 3.21.0 2017-10-24 18:55:49
Enter ".help" for usage hints.
sqlite> .log stdout
sqlite> create table test (test integer); 
sqlite> .tables
test
sqlite> pragma journal_mode=wal;
wal
sqlite> .tables
(5386) os_unix.c:34923: (22) mmap(/root/testdb-shm) - Invalid argument
(5386) statement aborts at 24: [SELECT name FROM "main".sqlite_master  WHERE type IN ('table','view')   AND name NOT LIKE 'sqlite_%'   AND name LIKE ?1 ORDER BY 1] disk I/O error
Error: disk I/O error
6

After a quick Google search, I would say it doesn't...

8

I created separated ubifs partition and now this application works. But it has two modes of operation and I started the simplest one (running as a root).
I would like to run it in a container, but I am getting an error

starting container process caused "process_linux.go:345: container init caused \"rootfs_linux.go:62: mounting \\\"proc\\\" to rootfs \\\"/usrdata/greengrass/ggc/packages/1.7.0/rootfs/merged\\\" at \\\"/proc\\\" caused \\\"operation not permitted\\\"\

Do you know if this error is related to nesting overlay partitions (openwrt has already overlayfs on /) or is it something directly related to /proc?
I tried to remount proc:

mount -o remount,rw,relatime /proc

but mount point didn't change.

My mount points:

dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
cgroup on /sys/fs/cgroup type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset,cpu,cpuacct,blkio,memory,devices,freezer,net_cls,perf_event,net_prio,pids,debug)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/mtdblock7 on /overlay type jffs2 (rw,noatime)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
/dev/mtdblock5 on /mnt type jffs2 (rw,relatime)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
ubi0:usrdata on /usrdata type ubifs (rw,relatime)