linux-kernel - Re: [PATCH RFC 3/4] mm, slab: add static key for should

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Date: Sun, 2 Jun 2024 12:12:29 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Akinobu Mita <akinobu.mita@...il.com>, Christoph Lameter <cl@...ux.com>, 
	David Rientjes <rientjes@...gle.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>, 
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, "David S. Miller" <davem@...emloft.net>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Mark Rutland <mark.rutland@....com>, Jiri Olsa <jolsa@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	bpf <bpf@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 3/4] mm, slab: add static key for should_failslab()

On Sat, Jun 1, 2024 at 1:57 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 5/31/24 6:43 PM, Alexei Starovoitov wrote:
> > On Fri, May 31, 2024 at 2:33 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>         might_alloc(flags);
> >>
> >> -       if (unlikely(should_failslab(s, flags)))
> >> -               return NULL;
> >> +       if (static_branch_unlikely(&should_failslab_active)) {
> >> +               if (should_failslab(s, flags))
> >> +                       return NULL;
> >> +       }
> >
> > makes sense.
> > Acked-by: Alexei Starovoitov <ast@...nel.org>
>
> Thanks :) but please note the cover letter where I explain how I need help
> with the bpftrace side (and ftrace, but that seems sorted). Without that
> part, bpftrace will silently stop doing the injection as the static key will
> remain disabled.

Right. That part was clear. Once this set lands we can add
static key on/off logic either in the kernel directly, or in libbpf.
In the kernel is certainly cleaner.
How will ftrace handle it? I couldn't figure it out from this set.
Ideally key toggle should be a part of generic kprobe attach logic
and not bpf specific, then both bpf and kprobe will work.