linux-kernel - Re: [RFC][PATCH 01/14 v2] function_graph: Convert ret

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Date:   Tue, 4 Jun 2019 05:04:07 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC][PATCH 01/14 v2] function_graph: Convert ret_stack to a
 series of longs

On Mon, 3 Jun 2019 20:30:49 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> > >   
> > > > +#define SHADOW_STACK_SIZE (PAGE_SIZE)    
> > > 
> > > Do we really need that big a shadow stack?  
> > 
> > Well, this is a sticky point. I allow up to 16 users at a time
> > (although I can't imagine more than 5, but you never know), and each
> > user adds a long and up to 4 more words (which is probably unlikely
> > anyway). And then we can have deep call stacks (we are getting deeper
> > each release it seems).
> > 
> > I figured, I start with a page size, and then in the future we can make
> > it dynamic, or shrink it if it proves to be too much.  
> 
> I'd prefer dynamic allocation, based on the number of users or actual
> stack starvation.

As stated, it's something we can improve on in the future. I'll
probably be pushing out this series for linux-next, and then we can
incrementally improve it.

First on my list is to add a REGS version of function_graph such that
kretprobes can use it ;-)

-- Steve