Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe trampolines

[Andrii Nakryiko]

On Tue, Nov 19, 2024 at 7:14 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Mon, Nov 18, 2024 at 10:05:41PM -0800, Andrii Nakryiko wrote:
> > On Sat, Nov 16, 2024 at 1:44 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > >
> > > On Thu, Nov 14, 2024 at 03:44:14PM -0800, Andrii Nakryiko wrote:
> > > > On Tue, Nov 5, 2024 at 5:35 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > > > >
> > > > > Adding interface to add special mapping for user space page that will be
> > > > > used as place holder for uprobe trampoline in following changes.
> > > > >
> > > > > The get_tramp_area(vaddr) function either finds 'callable' page or create
> > > > > new one.  The 'callable' means it's reachable by call instruction (from
> > > > > vaddr argument) and is decided by each arch via new arch_uprobe_is_callable
> > > > > function.
> > > > >
> > > > > The put_tramp_area function either drops refcount or destroys the special
> > > > > mapping and all the maps are clean up when the process goes down.
> > > > >
> > > > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > > > > ---
> > > > >  include/linux/uprobes.h |  12 ++++
> > > > >  kernel/events/uprobes.c | 141 ++++++++++++++++++++++++++++++++++++++++
> > > > >  kernel/fork.c           |   2 +
> > > > >  3 files changed, 155 insertions(+)
> > > > >
> > > > > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > > > > index be306028ed59..222d8e82cee2 100644
> > > > > --- a/include/linux/uprobes.h
> > > > > +++ b/include/linux/uprobes.h
> > > > > @@ -172,6 +172,15 @@ struct xol_area;
> > > > >
> > > > >  struct uprobes_state {
> > > > >         struct xol_area         *xol_area;
> > > > > +       struct hlist_head       tramp_head;
> > > > > +       struct mutex            tramp_mutex;
> > > > > +};
> > > > > +
> > > > > +struct tramp_area {
> > > > > +       unsigned long           vaddr;
> > > > > +       struct page             *page;
> > > > > +       struct hlist_node       node;
> > > > > +       refcount_t              ref;
> > > >
> > > > nit: any reason we are unnecessarily trying to save 4 bytes on
> > > > refcount (and we don't actually, due to padding)
> > >
> > > hum, I'm not sure what you mean.. what's the alternative?
> >
> > atomic64_t ?
>
> hum, just because we have extra 4 bytes padding? we use refcount_t
> on other places so seems like better fit to me

My (minor) concern was that tramp_area is a very long-living object
that can be shared across huge amounts of uprobes, and 2 billion
uprobes is a lot, but still a number that one can, practically
speaking, reach (with enough effort). And so skimping on refcount to
save 4 bytes for something that we have one or two per *some*
processes seemed (and still seems) like wrong savings to pursue.

>
> jirka