I have just installed trunk Openwrt firmware on my Tplink TL-MR3420 and etup an extroot.
Everything is working fine except when i try to install anything i get errors.

Package kmod-usb2 (4.1.16-1) installed in root is up to date.
Configuring kmod-usb-storage-extras.
Collected errors:
* xsystem: sh: vfork: Out of memory.
* pkg_run_script: package "kmod-usb-storage-extras" postinst script returned status -1.
* opkg_configure: kmod-usb-storage-extras.postinst returned -1.

please help me. I am new with openwrt. Thanks

EDIT

I tried to reinstall it after removing it and i got a new error.

Collected errors:
* gz_open: fork: Out of memory.
* opkg_install_pkg: Failed to unpack control files from /tmp/opkg-Ohmkjn/kmod-usb-storage-extras_4.1.16-1_ar71xx.ipk.
* opkg_install_cmd: Cannot install package kmod-usb-storage-extras.

(Last edited by redd_llining on 9 Feb 2016, 17:29)

Are you sure that it has space left to install those packages? I m not a professional at Linux nor OpenWrt, but I think that /tmp is full or simply that the memory (ram) is full.
First, reboot and try again. If that doesn't help:
Please type in the console "df -h" without the quotes and paste what you get.

(Last edited by Fusseldieb on 9 Feb 2016, 19:53)

thanks for reply, but no memory is not full. The extroot is attached and it shows 500+ mb of memory.

root@OpenWrt:/etc/config# df -h
Filesystem                Size         Used   Available     Use%    Mounted on
/dev/root                 2.0M        2.0M         0          100%       /rom
tmpfs                      13.8M      668.0K    13.1M       5%        /tmp
/dev/sda1                557.3M     1.5M     521.9M       0%       /overlay
overlayfs:/overlay     557.3M     1.5M     521.9M       0%      /
tmpfs                       512.0K        0        512.0K       0%      /dev

N.B. /dev/sda1 is mounted on root ('/').

And perhaps one might Read The Fine Wiki -

What is the partition type of /dev/sda1? Hopefully not vfat.
Post /etc/config/fstab.