memcg: cat: memory.memsw.* : Operation not supported

[Zhouping Liu <zliu@redhat.com>]

hi, all

when I used memory cgroup in latest mainline, the following error occurred:

# mount -t cgroup -o memory xxx /cgroup/
# ll /cgroup/memory.memsw.*
-rw-r--r--. 1 root root 0 Jun 26 23:17 /cgroup/memory.memsw.failcnt
-rw-r--r--. 1 root root 0 Jun 26 23:17 /cgroup/memory.memsw.limit_in_bytes
-rw-r--r--. 1 root root 0 Jun 26 23:17 /cgroup/memory.memsw.max_usage_in_bytes
-r--r--r--. 1 root root 0 Jun 26 23:17 /cgroup/memory.memsw.usage_in_bytes
# cat /cgroup/memory.memsw.*
cat: /cgroup/memory.memsw.failcnt: Operation not supported
cat: /cgroup/memory.memsw.limit_in_bytes: Operation not supported
cat: /cgroup/memory.memsw.max_usage_in_bytes: Operation not supported
cat: /cgroup/memory.memsw.usage_in_bytes: Operation not supported

I'm confusing why it can't read memory.memsw.* files.

as commit:a42c390cfa0c said, CGROUP_MEM_RES_CTLR_SWAP_ENABLED and
swapaccount kernel parameter control memcg swap accounting,
but I confirmed the two options all don't be set:

# cat /usr/lib/modules/3.5.0-rc4+/source/.config | grep CGROUP_MEM
CONFIG_CGROUP_MEM_RES_CTLR=y
CONFIG_CGROUP_MEM_RES_CTLR_SWAP=y
# CONFIG_CGROUP_MEM_RES_CTLR_SWAP_ENABLED is not set
CONFIG_CGROUP_MEM_RES_CTLR_KMEM=y
# cat /proc/cmdline 
BOOT_IMAGE=/vmlinuz-3.5.0-rc4+ root=/dev/mapper/vg_amd--pike--06-lv_root ro rd.lvm.lv=vg_amd-pike-06/lv_swap rd.md=0 LANG=en_US.UTF-8 console=ttyS0,115200n81 KEYTABLE=us SYSFONT=True rd.luks=0 rd.dm=0 rd.lvm.lv=vg_amd-pike-06/lv_root

so I have two problems here:
 1. when kernel neither set 'CONFIG_CGROUP_MEM_RES_CTLR_SWAP_ENABLED' nor 'swapaccount' options,
    why memcg have memory.memsw.* files ?

 2. why we can't read memory.memsw.* ?

Addition info:
when I open CONFIG_CGROUP_MEM_RES_CTLR_SWAP_ENABLED option, the above issues are gone.
also I tested v3.4.0, there aren't the two issues, so please take a look.

-- 
Thanks,
Zhouping

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