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Subject: Re: [PATCH] perf_events: fix errors path in perf_output_begin()
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
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Date: Mon, 17 May 2010 14:03:00 +0200
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On Mon, 2010-05-17 at 13:56 +0200, Stephane Eranian wrote:
> >> +     if (!data->nr_pages) {
> >> +             atomic_inc(&data->lost);
> >> +             goto out;
> >> +     }


> Well, nr_pages = 0 means all you have is the sampling buffer
> header page. You cannot save any sample, so you  actually
> loose everything. Perf reports a dozen samples but those
> are synthetic. The kernel cannot reject an mmap() for
> just one page because of the remapped counters, thus it
> has to handle this gracefully and at least report the lost
> samples.

So you want to preserve this state for when you munmap() and mmap()
again? The only user of data->lost is writing the PERF_RECORD_LOST
event, which only ever happens when you have pages, so counting it when
there's no pages seems futile.