netdev - Re: [PATCH] datapath: Avoid using stack larger than 1024.

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Date:   Tue, 27 Jun 2017 14:19:38 -0700
From:   Pravin Shelar <pshelar@....org>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] datapath: Avoid using stack larger than 1024.

On Tue, Jun 27, 2017 at 2:20 AM, Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> When compiling OvS-master on 4.4.0-81 kernel,
> there is a warning:
>
>     CC [M]  /root/ovs/datapath/linux/datapath.o
>     /root/ovs/datapath/linux/datapath.c: In function
>     ‘ovs_flow_cmd_set’:
>     /root/ovs/datapath/linux/datapath.c:1221:1: warning:
>     the frame size of 1040 bytes is larger than 1024 bytes
>     [-Wframe-larger-than=]
>
> This patch use kmalloc to malloc mem for sw_flow_mask and
> avoid using stack.
>
> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
> ---
>  net/openvswitch/datapath.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> index c85029c..da8cd68 100644
> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
> @@ -1107,7 +1107,7 @@ static int ovs_flow_cmd_set(struct sk_buff *skb, struct genl_info *info)
>         struct ovs_header *ovs_header = info->userhdr;
>         struct sw_flow_key key;
>         struct sw_flow *flow;
> -       struct sw_flow_mask mask;
> +       struct sw_flow_mask *mask;
>         struct sk_buff *reply = NULL;
>         struct datapath *dp;
>         struct sw_flow_actions *old_acts = NULL, *acts = NULL;
> @@ -1120,7 +1120,11 @@ static int ovs_flow_cmd_set(struct sk_buff *skb, struct genl_info *info)
>
>         ufid_present = ovs_nla_get_ufid(&sfid, a[OVS_FLOW_ATTR_UFID], log);
>         if (a[OVS_FLOW_ATTR_KEY]) {
> -               ovs_match_init(&match, &key, true, &mask);
> +               mask = kmalloc(sizeof(struct sw_flow_mask), GFP_KERNEL);
> +               if (!mask)
> +                       return -ENOMEM;
> +
> +               ovs_match_init(&match, &key, true, mask);

Rather than allocating mask object and freeing it at the end, can you
factor out code which needs mask into new function to save some stack
space in this function?

>                 error = ovs_nla_get_match(net, &match, a[OVS_FLOW_ATTR_KEY],
>                                           a[OVS_FLOW_ATTR_MASK], log);
>         } else if (!ufid_present) {
> @@ -1141,7 +1145,7 @@ static int ovs_flow_cmd_set(struct sk_buff *skb, struct genl_info *info)
>                 }
>
>                 acts = get_flow_actions(net, a[OVS_FLOW_ATTR_ACTIONS], &key,
> -                                       &mask, log);
> +                                       mask, log);
>                 if (IS_ERR(acts)) {
>                         error = PTR_ERR(acts);
>                         goto error;
> @@ -1216,6 +1220,7 @@ err_unlock_ovs:
>  err_kfree_acts:
>         ovs_nla_free_flow_actions(acts);
>  error:
> +       kfree(mask);

mask free is missing in usual, no error case.