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

[Jan Kara]

On Wed 29-08-07 12:00:07, Balbir Singh wrote:
> Andrew Morton wrote:
> > On Tue, 28 Aug 2007 16:13:18 +0200 Jan Kara <jack at suse.cz> wrote:
> >>   I'm sending rediffed patch implementing sending of quota messages via netlink
> >> interface (some rationale in patch description). I've already posted it to
> >> LKML some time ago and there were no objections, so I guess it's fine to put
> >> it to -mm. Andrew, would you be so kind? Thanks.
> >>   Userspace deamon reading the messages from the kernel and sending them to
> >> dbus and/or user console is also written (it's part of quota-tools). The
> >> only remaining problem is there are a few changes needed to libnl needed for
> >> the userspace daemon. They were basically acked by the maintainer but it
> >> seems he has not merged the patches yet. So this will take a bit more time.
> >>
> > 
> > So it's a new kernel->userspace interface.
> > 
> > But we have no description of the interface :(
> > 
> 
> And could we have some description of the context under which all the message
> exchanges take place. When are these messages sent out -- what event
> is the user space notified of?
  The user is notified about either exceeding his quota softlimit or
reaching hardlimit. If you are interested in more details, please ask.

> >> +/* Send warning to userspace about user which exceeded quota */
> >> +static void send_warning(const struct dquot *dquot, const char warntype)
> >> +{
> >> +	static unsigned long seq;
> >> +	struct sk_buff *skb;
> >> +	void *msg_head;
> >> +	int ret;
> >> +
> >> +	skb = genlmsg_new(QUOTA_NL_MSG_SIZE, GFP_NOFS);
> >> +	if (!skb) {
> >> +		printk(KERN_ERR
> >> +		  "VFS: Not enough memory to send quota warning.\n");
> >> +		return;
> >> +	}
> >> +	msg_head = genlmsg_put(skb, 0, seq++, &quota_genl_family, 0, QUOTA_NL_C_WARNING);
> >> +	if (!msg_head) {
> >> +		printk(KERN_ERR
> >> +		  "VFS: Cannot store netlink header in quota warning.\n");
> >> +		goto err_out;
> 
> One problem, we've been is losing notifications. It does not happen for us
> due to the cpumask interface (which allows us to have parallel sockets
> for each cpu or a set of cpus). How frequent are your notifications?
  Quite infrequent... Users won't exceed their quotas too often :).

> >> +	}
> >> +	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?

> >> +	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.
> 
> > Does it have (or need) reserved-set-to-zero space for expansion?  Again,
> > hard to tell..
> > 
> > I guess it's OK to send a major and minor out of the kernel like this. 
> > What's it for?  To represent a filesytem?  I wonder if there's a more
> > modern and useful way of describing the fs.  Path to mountpoint or
> > something?
> > 
> > I suspect the namespace virtualisation guys would be interested in a new
> > interface which is sending current->user->uid up to userspace.  uids are
> > per-namespace now.  What are the implications?  (cc's added)
> 
> 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.

									Honza
-- 
Jan Kara <jack at suse.cz>
SuSE CR Labs
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