netdev - Re: [PATCH net v4 2/2] net/mlx5e: Don't overwrite pedit action when multiple pedit used

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Date:   Tue, 29 Jan 2019 17:44:12 +0200
From:   Or Gerlitz <gerlitz.or@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Linux Netdev List <netdev@...r.kernel.org>,
        Or Gerlitz <ogerlitz@...lanox.com>
Subject: Re: [PATCH net v4 2/2] net/mlx5e: Don't overwrite pedit action when
 multiple pedit used

On Mon, Jan 28, 2019 at 6:18 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> On Mon, Jan 28, 2019 at 11:34 PM Or Gerlitz <gerlitz.or@...il.com> wrote:
> > On Mon, Jan 28, 2019 at 2:10 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> > > On Mon, Jan 28, 2019 at 12:40 AM Or Gerlitz <gerlitz.or@...il.com> wrote:
> > > >
> > > > On Sun, Jan 27, 2019 at 1:06 PM <xiangxia.m.yue@...il.com> wrote:
> > > > > From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> > > > >
> > > > > In some case, we may use multiple pedit actions to modify packets.
> > > > > The command shown as below: the last pedit action is effective.
> > > >
> > > > > @@ -2073,7 +2076,8 @@ static int alloc_mod_hdr_actions(struct mlx5e_priv *priv,
> > > > >         if (!parse_attr->mod_hdr_actions)
> > > > >                 return -ENOMEM;
> > > > >
> > > > > -       parse_attr->num_mod_hdr_actions = max_actions;
> > > > > +       parse_attr->max_mod_hdr_actions = max_actions;
> > > > > +       parse_attr->num_mod_hdr_actions = 0;
> > > >
> > > > why would we want to do this zeroing? what purpose does it serve?
> > > Because we use the num_mod_hdr_actions to store the number of actions
> > > we have parsed,
> > > and when we alloc it, we init it 0 as default.
> > >
> > > > On a probably related note, I suspect that the patch broke the caching
> > > > we do for modify header contexts, see mlx5e_attach_mod_hdr where we
> > > > look if a given set of modify header operations already has hw modify header
> > > > context and we use it.
> > > >
> > > > To test that, put two tc rules with different matching but same set of
> > > > modify header
> > > > (pedit) actions and see that only one modify header context is used.
> >
> > > The patch does't break the cache, I think that different matching may
> > > share the same set of pedit actions.
> >
> > I suspect it does break it.. at [1]  we have this code for the cache lookup:
> >
> > num_actions  = parse_attr->num_mod_hdr_actions;
> > [..]
> > key.actions = parse_attr->mod_hdr_actions;
> > key.num_actions = num_actions;
> >
> > hash_key = hash_mod_hdr_info(&key);
> >
> > so we are doing the cached insertion and lookup with
> > parse_attr->num_mod_hdr_actions
> > which was zeroed along the way and not accounting for the full set of
> > pedit actions, agree?

> not really, before calling the  mlx5e_attach_mod_hdr,  we have call
> the offload_pedit_fields that will
> update the attr->num_mod_hdr_actions that indicate  how many pedit
> action we parsed.

ok, got you, so why do we need this line

 parse_attr->num_mod_hdr_actions = 0;

in alloc_mod_hdr_actions()? this should be zero
by kzalloc somewhere, it got to confuse me..

I suggest to remove this zeroing, otherwise the patch LGTM, once you fix it

Reviewed-by: Or Gerlitz <ogerlitz@...lanox.com>