[libvirt] [PATCH] LXC: create directory /dev/shm automatically

[Kamezawa Hiroyuki]

(2013/01/29 11:16), Gao feng wrote:
> On 2013/01/29 07:52, Kamezawa Hiroyuki wrote:
>> (2013/01/28 19:36), Daniel P. Berrange wrote:
>>> On Mon, Jan 28, 2013 at 02:37:11PM +0800, Gao feng wrote:
>>>> Now we mount /dev as tmpfs and haven't created directory
>>>> /dev/shm,so the glibc api such as shm_open/sem_open will
>>>> create files under dir /dev.(since /dev is mounted as tmpfs)
>>>>
>>>> Through these api still useable in container,but this cause
>>>> directory /dev looks a little chaos.
>>>>
>>>> This patch create directory /dev/shm automatically,the files
>>>> created by shm_open/sem_open will stay in this directroy.
>>>>
>>>> Signed-off-by: Gao feng <gaofeng at cn.fujitsu.com>
>>>> ---
>>>>    src/lxc/lxc_container.c | 7 +++++++
>>>>    1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
>>>> index 497539c..5150564 100644
>>>> --- a/src/lxc/lxc_container.c
>>>> +++ b/src/lxc/lxc_container.c
>>>> @@ -586,6 +586,13 @@ static int lxcContainerMountBasicFS(bool pivotRoot,
>>>>                                     "devfs", "/dev", "tmpfs", opts);
>>>>                goto cleanup;
>>>>            }
>>>> +
>>>> +        VIR_DEBUG("create directory /dev/shm for POSIX shared memory and named semphore");
>>>> +        if (virFileMakePath("/dev/shm") < 0) {
>>>> +            virReportSystemError(errno, "%s",
>>>> +                                 _("Failed to mkdir /dev/shm"));
>>>> +            goto cleanup;
>>>> +        }
>>>>        }
>>>>
>>>>        rc = 0;
>>>
>>> I we probably want to mount a separate tmpfs on /dev/shm really, so we can
>>> do resource limits on /dev and /dev/shm separately.
>>
>> I agree.
>> BTW, if the user wants to tune limit of /dev/shm size, he need to add
>>
>> <filesystem type='ram'>
>>    <source usage='XXXXX'/>
>>    <target dir='/dev/shm'/>
>> </filesystem>
>>
>> ?
>>
>> How do you think suitable limit for default should be ?
>> half of memory limit of a container ?
>>
>>
>
> I think we needn't consider about this problem,The root user
> of the container should do this job.
>
> For libvirt lxc,we only need to limit the memory resource that
> the container uses.And we needn't setup any xml configuration too.
>
> Or maybe I misunderstand what you mean?
>

"How large you can make files on a fs" is different from memcg's limit.

IIUC, tmpfs's size is detemined as the half of system memory regardless of
the configuration of a domain. So,

- if a domain is enough big, shm's memory usage will hit tmpfs' limit before
   memcg's.
- if a domain is enough small, shm's memory usage will hit memcg's limit before
   tmpfs's.

I think it's complicated and hard to use from viewpoint of application in a
container.

IMHO, tmpfs's limit size should be same to <memory> tag at default.

Thanks,
-Kame