netdev - Re: [PATCH v2 net] af_unix: Do not call kmemdup() for init

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Date:   Mon, 27 Jun 2022 20:40:24 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH v2 net] af_unix: Do not call kmemdup() for init_net's
 sysctl table.

On Mon, Jun 27, 2022 at 8:30 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From:   Jakub Kicinski <kuba@...nel.org>
> Date:   Mon, 27 Jun 2022 10:58:59 -0700
> > On Sun, 26 Jun 2022 11:43:27 -0500 Eric W. Biederman wrote:
> > > Kuniyuki Iwashima <kuniyu@...zon.com> writes:
> > >
> > > > While setting up init_net's sysctl table, we need not duplicate the global
> > > > table and can use it directly.
> > >
> > > Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> > >
> > > I am not quite certain the savings of a single entry table justivies
> > > the complexity.  But the looks correct.
> >
> > Yeah, the commit message is a little sparse. The "why" is not addressed.
> > Could you add more details to explain the motivation?
>
> I was working on a series which converts UDP/TCP hash tables into per-netns
> ones like AF_UNIX to speed up looking up sockets.  It will consume much
> memory on a host with thousands of netns, but it can be waste if we do not
> have its protocol family's sockets.

For the record, I doubt we will accept such a patch (per net-ns
TCP/UDP hash tables)

>
> So, I'm now working on a follow-up series for AF_UNIX per-netns hash table
> so that we can change the size for a child netns by a sysctl knob:
>
>   # sysctl -w net.unix.child_hash_entries=128
>   # ip net add test  # created with the hash table size 128
>   # ip net exec test sh
>   # sysctl net.unix.hash_entries  # read-only
>   128
>
>   (The size for init_net can be changed via a new boot parameter
>    xhash_entries like uhash_entries/thash_entries.)
>
> While implementing that, I found that kmemdup() is called for init_net but
> TCP/UDP does not (See: ipv4_sysctl_init_net()).  Unlike IPv4, AF_UNIX does
> not have a huge sysctl table, so it cannot be a problem though, this patch
> is for consuming less memory and kind of consistency.  The reason I submit
> this seperately is that it might be better to have a Fixes tag.

I think that af_unix module can be unloaded.

Your patch will break the module unload operation.