Re: [PATCH] perf_events: fix errors path in perf_output_begin()

From: Peter Zijlstra
Date: Mon May 17 2010 - 08:03:43 EST

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.

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