[Devel] Re: [PATCH] Send quota messages via netlink

[Balbir Singh]

Jan Kara wrote:
>>>> +	}
>>>> +	ret = nla_put_u32(skb, QUOTA_NL_A_QTYPE, dquot->dq_type);
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	ret = nla_put_u64(skb, QUOTA_NL_A_EXCESS_ID, dquot->dq_id);
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	ret = nla_put_u32(skb, QUOTA_NL_A_WARNING, warntype);
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	ret = nla_put_u32(skb, QUOTA_NL_A_DEV_MAJOR,
>>>> +		MAJOR(dquot->dq_sb->s_dev));
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	ret = nla_put_u32(skb, QUOTA_NL_A_DEV_MINOR,
>>>> +		MINOR(dquot->dq_sb->s_dev));
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	ret = nla_put_u64(skb, QUOTA_NL_A_CAUSED_ID, current->user->uid);
>>>> +	if (ret)
>>>> +		goto attr_err_out;
>>>> +	genlmsg_end(skb, msg_head);
>>>> +
>> Have you looked at ensuring that the data structure works across 32 bit
>> and 64 bit systems (in terms of binary compatibility)? That's usually
>> a nice to have feature.
>   Generic netlink should take care of this - arguments are typed so it
> knows how much bits numbers have. So this should be no issue. Are there any
> other problems that you have in mind?
> 

Yes, but apart from that, if I remember Jamal Hadi's initial comments
on taskstats, he recommended that we align everything to 64 bit so
that the data is well aligned for 64 bit systems. You could also consider
creating a data structure, document it's members, align them and use
that to send out the data.

>>>> +	ret = genlmsg_multicast(skb, 0, quota_genl_family.id, GFP_NOFS);
>>>> +	if (ret < 0 && ret != -ESRCH)
>>>> +		printk(KERN_ERR
>>>> +			"VFS: Failed to send notification message: %d\n", ret);
>>>> +	return;
>>>> +attr_err_out:
>>>> +	printk(KERN_ERR "VFS: Failed to compose quota message: %d\n", ret);
>>>> +err_out:
>>>> +	kfree_skb(skb);
>>>> +}
>>>> +#endif
>>> This is it.  Normally netlink payloads are represented as a struct.  How
>>> come this one is built-by-hand?
>>>
>>> It doesn't appear to be versioned.  Should it be?
>>>
>> Yes, versioning is always nice and genetlink supports it.
>>

It would nice for you to use the versioning feature.

>> The memory controller or VM would also be interested in notifications
>> of OOM. At OLS this year interest was shown in getting OOM notifications
>> and allow the user space a chance to handle the notification and take
>> action (especially for containers). We already have containerstats for
>> containers (which I was planning to reuse), but I was told that we would
>> be interested in user space OOM notifications in general.

>  Generic netlink can be used to pass this information (although in OOM
> situation, it may be a bit hairy to get the network stack working...). But
> I guess it's not related to my patch.

We could have a pre-allocated buffer stored at startup and use that for
OOM notification. In the case of container OOM, we are likely to have
free global memory. Working towards an infrastructure so that anybody can
build on top of it and sending notifications on interesting events becomes
easier would be nice. We can reuse code that way and add fewer bugs :-)


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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