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

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Date:   Mon, 27 Jun 2022 22:04:07 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     David Miller <davem@...emloft.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jakub Kicinski <kuba@...nel.org>,
        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 10:00 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From:   Eric Dumazet <edumazet@...gle.com>
> Date:   Mon, 27 Jun 2022 21:36:18 +0200
> > On Mon, Jun 27, 2022 at 9:16 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > >
> > > From:   Eric Dumazet <edumazet@...gle.com>
> > > Date:   Mon, 27 Jun 2022 21:06:14 +0200
> > > > On Mon, Jun 27, 2022 at 8:59 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > > > >
> > > > > From:   Eric Dumazet <edumazet@...gle.com>
> > > > > Date:   Mon, 27 Jun 2022 20:40:24 +0200
> > > > > > 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)
> > > > >
> > > > > Is it because it's risky?
> > > >
> > > > Because it will be very expensive. TCP hash tables are quite big.
> > >
> > > Yes, so I'm wondering if changing the size by sysctl makes sense.  If we
> > > have per-netns hash tables, each table should have smaller amount of
> > > sockets and smaller size should be enough, I think.
> >
> > How can a sysctl be safely used if two different threads call "unshare
> > -n" at the same time ?
>
> I think two unshare are safe. Each of them reads its parent netns's sysctl
> knob.  Even when the parent is the same, they can read the same value.

How can one thread create a netns with a TCP ehash table with 1024 buckets,
and a second one create a netns with a TCP ehash table with 1 million
buckets at the same time,
if they share the same sysctl ???

>
> But I think we need READ_ONCE()/WRITE_ONCE() in such a sysctl.

Like all sysctls really.

  While
> creating a child netns, another one can change the value and there can be
> a data-race.  So we have to use custome handler and pass write/read handler
> as conv of do_proc_douintvec(), like do_proc_douintvec_conv_lockless().
>
> If there are some sysctls missing READ_ONCE/WRITE_ONCE(), I will add
> more general one, proc_douintvec_lockless().

Seriously, all sysctls can be set while being read. That is not something new.

>
>
> > > > [    4.917080] tcp_listen_portaddr_hash hash table entries: 65536
> > > > (order: 8, 1048576 bytes, vmalloc)
> > > > [    4.917260] TCP established hash table entries: 524288 (order: 10,
> > > > 4194304 bytes, vmalloc hugepage)
> > > > [    4.917760] TCP bind hash table entries: 65536 (order: 8, 1048576
> > > > bytes, vmalloc)
> > > > [    4.917881] TCP: Hash tables configured (established 524288 bind 65536)
> > > >
> > > >
> > > >
> > > > > IIRC, you said we need per netns table for TCP in the future.
> > > >
> > > > Which ones exactly ? I guess you misunderstood.
> > >
> > > I think this.
> > > https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=04c494e68a13
> >
> > "might" is very different than "will"
> >
> > I would rather use the list of time_wait, instead of adding huge
> > memory costs for hosts with hundreds of netns.
>
> Sorry, my bad.
> I would give it a try only for TIME_WAIT.