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From: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
To: David Ahern <dsahern@gmail.com>
Cc: mingo@redhat.com, acme@ghostprotocols.net, a.p.zijlstra@chello.nl,
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        linux-kernel@vger.kernel.org
Subject: Re: [BUG PATCH] perf: kvm - finding struct machine fails for PERF_RECORD_MMAP
In-Reply-To: <4F8369E6.5090608@gmail.com>
References: <20120409081835.10576.22018.stgit@abhimanyu.in.ibm.com> <4F8369E6.5090608@gmail.com>
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Date: Tue, 10 Apr 2012 09:10:33 +0530
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On Mon, 09 Apr 2012 16:59:50 -0600, David Ahern <dsahern@gmail.com> wrote:
> On 4/9/12 2:22 AM, Nikunj A. Dadhania wrote:
> > From: Nikunj A. Dadhania<nikunj@linux.vnet.ibm.com>
> >
> > Running 'perf kvm --host --guest --guestmount /tmp/guestmount record -a -g -- sleep 2'
> >
> > Was resulting in a segfault. For event type PERF_RECORD_MMAP,
> > event->ip.pid is being used in perf_session__find_machine_for_cpumode,
> > which is not correct.
> >
> > event->ip.pid happens to be 0 in this case and results in returning a
> > NULL machine object. Finally, access to self->pid in
> > machine__mmap_name, results in a segfault later.
> 
> I have tried unsuccessfully to recreate the segfault. All of the samples 
> I get are of type PERF_RECORD_SAMPLE. I do not see how an MMAP event can 
> be generated in a guest context.
> 
I can think of two places where it can happen:

1) perf_event__synthesize_kernel_mmap

        [...]
 	mmap_name = machine__mmap_name(machine, name_buff, sizeof(name_buff));
	if (machine__is_host(machine)) {
           ...
	} else {
		event->header.misc = PERF_RECORD_MISC_GUEST_KERNEL;
		if (machine__is_default_guest(machine))
			filename = (char *) symbol_conf.default_guest_kallsyms;
		else {
			sprintf(path, "%s/proc/kallsyms", machine->root_dir);
			filename = path;
		}
	}
        [...]
	event->mmap.header.type = PERF_RECORD_MMAP;
        [...]
        event->mmap.pid   = machine->pid;        

2) perf_event__synthesize_modules

        [...]
	if (machine__is_host(machine))
		event->header.misc = PERF_RECORD_MISC_KERNEL;
	else
		event->header.misc = PERF_RECORD_MISC_GUEST_KERNEL;
        [...]             
	event->mmap.header.type = PERF_RECORD_MMAP;
        [...]
        event->mmap.pid   = machine->pid;

BTW, i am testing this on latest -tip(498c911).

In my case, I am able to recreate this everytime. And have verified that
the event->header.type that I hit here is of type PERF_RECORD_MMAP.

Let me know if you want me to test out some debug patches.

Regards,
Nikunj