netdev - Re: [PATCH v2] sched: report if filter is too large to dump

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Message-ID: <20180221084240.GF4196@orbyte.nwl.cc>
Date:   Wed, 21 Feb 2018 09:42:40 +0100
From:   Phil Sutter <phil@....cc>
To:     Roman Kapl <code@...pl.cz>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v2] sched: report if filter is too large to dump

Hi Roman,

On Wed, Feb 21, 2018 at 09:38:52AM +0100, Roman Kapl wrote:
> On 02/21/2018 08:45 AM, Phil Sutter wrote:
> > On Mon, Feb 19, 2018 at 09:32:51PM +0100, Roman Kapl wrote:
> >> So far, if the filter was too large to fit in the allocated skb, the
> >> kernel did not return any error and stopped dumping. Modify the dumper
> >> so that it returns -EMSGSIZE when a filter fails to dump and it is the
> >> first filter in the skb. If we are not first, we will get a next chance
> >> with more room.
> >>
> >> I understand this is pretty near to being an API change, but the
> >> original design (silent truncation) can be considered a bug.
> >>
> >> Note: The error case can happen pretty easily if you create a filter
> >> with 32 actions and have 4kb pages. Also recent versions of iproute try
> >> to be clever with their buffer allocation size, which in turn leads to
> > I'm curious, what does it lead to? :)
> >
> > Thanks, Phil
> 
> tc will dump all filters up to the big one, then it will stop silently. 
> So it will seem as if you have less filters.
> 
> The new behavior is the same, but tc will at leas print the EMSGSIZE 
> error. It does not handle it in any other way.

I got that, yes. Though your commit message stops mid-sentence and I
wondered what the dynamic buffer allocation "in turn leads to".

Thanks, Phil