netdev - Re: [RFC PATCH] netlink: Increase netlink dump skb message size

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Date:	Fri, 29 Apr 2011 12:29:40 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	gregory.v.rose@...el.com
Cc:	netdev@...r.kernel.org, bhutchings@...arflare.com
Subject: Re: [RFC PATCH] netlink: Increase netlink dump skb message size

From: Greg Rose <gregory.v.rose@...el.com>
Date: Mon, 25 Apr 2011 15:01:57 -0700

> The message size allocated for rtnl info dumps was limited to a single page.
> This is not enough for additional interface info available with devices
> that support SR-IOV.  Check that the amount of data allocated is sufficient
> for the amount of data requested.
> 
> Signed-off-by: Greg Rose <gregory.v.rose@...el.com>

Actually, thinking about this problem some more I think your approach
is close to what we should actually do.

The only problem is that you've specialized netlink_dump() too much,
hide the issue in the rtnetlink device dumping code and provide
the necessary information via the control block.

1) Add some information to struct netlink_callback.

   	"u16	min_dump_alloc;"

   I think you can change "int family;" there to "u16 family;"
   so that there is no new space required to add this functionality.

2) In netlink_dump().

	int alloc_size;

	mutex_lock(nlk->cb_mutex);
	cb = nlk->cb;
	if (cb == NULL) {
		...
	}
	alloc_size = max_t(int, cb->min_dump_alloc, NLMSG_GOODSIZE);
	skb = sock_rmalloc(sk, alloc_size, 0, GFP_KERNEL);
	if (!skb) {
	...

3) In net/core/rtnetlink.c add a new method pointer to struct rtnl_link,
   "u16 (*calc_min_dump_alloc)(struct sk_buff *);"
   Add an implementation for the RTM_GETLINK slot.

   This function does the logic to compute the maximum space needed
   for the devices currently configured, as you hacked directly into
   the netlink_dump() logic in your match.

4) Make netlink_dump_start() take a new argument, this is where you
   pass the new min_dump_alloc value that will get stored in the
   newly allocated callback object.

	calcit = rtnl_get_calcit(family, type);

   	min_dump_alloc = 0;
	if (calcit)
		min_dump_alloc = calcit(skb);
	err = netlink_dump_start(rtnl, skb, nlh, dumpit, min_dump_alloc, NULL);

Anyways, something like that.

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