linux-kernel - Re: [PATCH bpf-next v2 27/35] bpf: eliminate rlimit-based memory accounting infra for bpf maps

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Message-ID: <20200728190830.GB410810@carbon.DHCP.thefacebook.com>
Date:   Tue, 28 Jul 2020 12:08:30 -0700
From:   Roman Gushchin <guro@...com>
To:     Song Liu <song@...nel.org>
CC:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 27/35] bpf: eliminate rlimit-based memory
 accounting infra for bpf maps

On Mon, Jul 27, 2020 at 11:06:42PM -0700, Song Liu wrote:
> On Mon, Jul 27, 2020 at 10:58 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Mon, Jul 27, 2020 at 10:47 PM Song Liu <song@...nel.org> wrote:
> > >
> > > On Mon, Jul 27, 2020 at 12:26 PM Roman Gushchin <guro@...com> wrote:
> > > >
> > > > Remove rlimit-based accounting infrastructure code, which is not used
> > > > anymore.
> > > >
> > > > Signed-off-by: Roman Gushchin <guro@...com>
> > > [...]
> > > >
> > > >  static void bpf_map_put_uref(struct bpf_map *map)
> > > > @@ -541,7 +484,7 @@ static void bpf_map_show_fdinfo(struct seq_file *m, struct file *filp)
> > > >                    "value_size:\t%u\n"
> > > >                    "max_entries:\t%u\n"
> > > >                    "map_flags:\t%#x\n"
> > > > -                  "memlock:\t%llu\n"
> > > > +                  "memlock:\t%llu\n" /* deprecated */
> > >
> > > I am not sure whether we can deprecate this one.. How difficult is it
> > > to keep this statistics?
> > >
> >
> > It's factually correct now, that BPF map doesn't use any memlock memory, no?

Right.

> 
> I am not sure whether memlock really means memlock for all users... I bet there
> are users who use memlock to check total memory used by the map.

But this is just the part of struct bpf_map, so I agree with Andrii,
it's a safe check.

> 
> >
> > This is actually one way to detect whether RLIMIT_MEMLOCK is necessary
> > or not: create a small map, check if it's fdinfo has memlock: 0 or not
> > :)
> 
> If we do show memlock=0, this is a good check...

The only question I have if it's worth checking at all? Bumping the rlimit
is a way cheaper operation than creating a temporarily map and checking its
properties.

So is there any win in comparison to just leaving the userspace code* as it is
for now?

* except runqslower and samples