linux-kernel - Re: [PATCH v5 bpf-next 1/3] bpf: Add kmem

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Message-ID: <CAEf4Bzarmpop6o9WwjKQpkUdUH=UWY9e+xBe4cg040pdpwz9AA@mail.gmail.com>
Date: Fri, 11 Oct 2024 12:43:44 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Namhyung Kim <namhyung@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, 
	Joonsoo Kim <iamjoonsoo.kim@....com>, Vlastimil Babka <vbabka@...e.cz>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	linux-mm <linux-mm@...ck.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, Kees Cook <kees@...nel.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v5 bpf-next 1/3] bpf: Add kmem_cache iterator

On Fri, Oct 11, 2024 at 12:41 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Oct 11, 2024 at 11:44 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Thu, Oct 10, 2024 at 4:25 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > +struct bpf_iter__kmem_cache {
> > > +       __bpf_md_ptr(struct bpf_iter_meta *, meta);
> > > +       __bpf_md_ptr(struct kmem_cache *, s);
> > > +};

BTW, do we want/need to define an open-coded iterator version of this,
so that this iteration can be done from other BPF programs? Seems like
it has to be a sleepable BPF program, but that's probably fine?

> >
> > Just noticed this.
> > Not your fault. You're copy pasting from bpf_iter__*.
> > It looks like tech debt.
> >
> > Andrii, Song,
> >
> > do you remember why all iters are using this?
>
> I don't *know*, but I suspect we are doing this because of 32-bit host
> architecture. BPF-side is always 64-bit, so to make memory layout
> inside the kernel and in BPF programs compatible we have to do this
> for pointers, no?
>
> > __bpf_md_ptr() wrap was necessary in uapi/bpf.h,
> > but this is kernel iters that go into vmlinux.h
> > It should be fine to remove them all and
> > progs wouldn't need to do the ugly dance of:
> >
> > #define bpf_iter__ksym bpf_iter__ksym___not_used
> > #include "vmlinux.h"
> > #undef bpf_iter__ksym
>
> I don't think __bpf_md_ptr is why we are doing this ___not_used dance.
> At some point we probably didn't want to rely on having the very
> latest vmlinux.h available in BPF selftests, so we chose to define
> local versions of all relevant context types.
>
> I think we can drop all that ___not_used dance regardless (and remove
> local definitions in progs/bpf_iter.h).