Re: [PATCH v2 net] af_unix: Do not call kmemdup() for init_net's sysctl table.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 27 Jun 2022 22:04:07 +0200
> On Mon, Jun 27, 2022 at 10:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > From:   Eric Dumazet <edumazet@google.com>
> > Date:   Mon, 27 Jun 2022 21:36:18 +0200
> > > On Mon, Jun 27, 2022 at 9:16 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > > >
> > > > From:   Eric Dumazet <edumazet@google.com>
> > > > Date:   Mon, 27 Jun 2022 21:06:14 +0200
> > > > > On Mon, Jun 27, 2022 at 8:59 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > > > > >
> > > > > > From:   Eric Dumazet <edumazet@google.com>
> > > > > > Date:   Mon, 27 Jun 2022 20:40:24 +0200
> > > > > > > On Mon, Jun 27, 2022 at 8:30 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > > > > > > >
> > > > > > > > From:   Jakub Kicinski <kuba@kernel.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@amazon.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@xmission.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 ???

Oh, I undertood.
In the example, I added net.unix.hash_entries so we can confirm if the size
is intended one, but yes, checking it and recreating netns is crazy...

  # 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

Do you have good idea?


> 
> >
> > 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.

Ok, I added that on TODO.