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Date:   Wed, 29 Apr 2020 12:52:45 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Joerg Roedel <jroedel@suse.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shile Zhang <shile.zhang@linux.alibaba.com>,
        Andy Lutomirski <luto@amacapital.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tzvetomir Stoyanov <tz.stoyanov@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [RFC][PATCH] x86/mm: Sync all vmalloc mappings before
 text_poke()
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On Wed, 29 Apr 2020 18:20:26 +0200
Joerg Roedel <jroedel@suse.de> wrote:

> On Wed, Apr 29, 2020 at 06:17:47PM +0200, Joerg Roedel wrote:
> > On Wed, Apr 29, 2020 at 10:07:31AM -0400, Steven Rostedt wrote:  
> > > Talking with Mathieu about this on IRC, he pointed out that my code does
> > > have a vzalloc() that is called:
> > > 
> > > in trace_pid_write()
> > > 
> > > 	pid_list->pids = vzalloc((pid_list->pid_max + 7) >> 3);
> > > 
> > > This is done when -P1,2 is on the trace-cmd command line.  
> > 
> > And that buffer is written to at any function entry?  
> 
> What I meant to say, is it possible that the page-fault handler does not
> complete because at its beginning it calls into trace-code and faults
> again on the same address?
> 

It should be read only at sched_switch.

Basically, it's a big bitmask, where each bit represents a possible process
id (can be 2 gigs if we allow all positive ints!).

Then, it is only written when setting it up. Bits 1 and 2 are set here
(-P1,2). At context switch, next->pid is checked against this bitmask, and
if it is set, it means we should allow this process to be traced.

This mask should only be accessed at sched_switch time, not at other times.
And it may read any possible page in that mask depending on the process id
of the next task to be scheduled in.

-- Steve