Re: [PATCH 1/2] perf: Add munmap callback

From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: "Liang, Kan" <kan.liang@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bp@alien8.de>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 1/2] perf: Add munmap callback
Date: Thu, 25 Oct 2018 02:23:12 +0200	[thread overview]
Message-ID: <20181025002312.GS3109@worktop.c.hoisthospitality.com> (raw)
In-Reply-To: <CABPqkBQEp9SLQuAonW7-c9HzH4bSgUyJGJ5ZZNXHy-0phHvvZQ@mail.gmail.com>

On Wed, Oct 24, 2018 at 12:30:52PM -0700, Stephane Eranian wrote:
> Hi,
> 
> On Wed, Oct 24, 2018 at 8:12 AM <kan.liang@linux.intel.com> wrote:
> >
> > From: Kan Liang <kan.liang@linux.intel.com>
> >
> > To calculate the physical address, perf needs to walk the pages tables.
> > The related mapping may has already been removed from pages table in
> > some cases (e.g. large PEBS). The virtual address recorded in the first
> > PEBS records may already be unmapped before draining PEBS buffers.
> >
> > Add a munmap callback to notify the PMU of any unmapping, which only be
> > invoked when the munmap is implemented.
> >
> The need for this new record type extends beyond physical address conversions
> and PEBS. A long while ago, someone reported issues with symbolization related
> to perf lacking munmap tracking. It had to do with vma merging. I think the
> sequence of mmaps was as follows in the problematic case:
> 1.   addr1 = mmap(8192);
> 2.   munmap(addr1 + 4096, 4096)
> 3.   addr2 = mmap(addr1+4096, 4096)
> 
> If successful, that yields addr2 = addr1 + 4096 (could also get the
> same without forcing the address).

That is actually a different problem. And you're right, we never did fix
that.

I agree with you that Kan's Changelog is somewhat cryptic; it took me at
least 3 times reading and looking at what the patches actually do to
understand :/