linux-kernel - Re: [PATCH] tracing/user_events: Run BPF program if attached

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Date:   Tue, 16 May 2023 17:56:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Beau Belgrave <beaub@...ux.microsoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        David Vernet <void@...ifault.com>, dthaler@...rosoft.com,
        brauner@...nel.org, hch@...radead.org
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached

On Tue, May 16, 2023 at 5:36 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, May 15, 2023 at 12:24:07PM -0700, Beau Belgrave wrote:
> > > >
> > > >   ret = pin_user_pages_remote(mm->mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT,
> > > >                               &page, NULL, NULL);
> > >
> > > ... which will call pin_user_pages_remote() in RCU CS.
> > > This looks buggy, since pin_user_pages_remote() may schedule.
> > >
> >
> > If it's possible to schedule, I can change this to cache the probe
> > callbacks under RCU then drop it. However, when would
> > pin_user_pages_remote() schedule with FOLL_NOFAULT?
>
> Are you saying that passing FOLL_NOFAULT makes it work in atomic context?

Absolutely not.

It may not fault missing pages in, but that does *not* make it atomic.

That code depends on all the usual MM locking, and it does not work at
all in the same way that "pagefault_disable()" does, for example. That
will fail on any fault and never take locks, and is designed to work
in atomic contexts. Very different.

So no, don't think you can call pin_user_pages_remote() or any other
GUP function from atomic context.

We do have "get_user_page[s]_fast_only()" and that is the only version
of GUP that is actually lock-free.

Also, just FYI, those special gup_user*fast_only()" functions simply
will not work on some architectures at all.

               Linus