Re: [PATCH v2 1/3] Revert "perf session: Fix decompression of PERF_RECORD_COMPRESSED records"

From: Petr Malat <oss@malat.biz>
To: Jiri Olsa <jolsa@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	Alexey Budankov <alexey.budankov@linux.intel.com>
Subject: Re: [PATCH v2 1/3] Revert "perf session: Fix decompression of PERF_RECORD_COMPRESSED records"
Date: Mon, 30 Nov 2020 12:40:20 +0100	[thread overview]
Message-ID: <20201130114020.GA29476@ntb.petris.klfree.czf> (raw)
In-Reply-To: <20201124181519.GA29264@ntb.petris.klfree.czf>

Hi Jiří,
were you able to reproduce the issue? I may also upload perf-archive
if that would help.
  Petr

On Tue, Nov 24, 2020 at 07:15:19PM +0100, Petr Malat wrote:
> Hi!
> On Tue, Nov 24, 2020 at 03:36:45PM +0100, Jiri Olsa wrote:
> > On Tue, Nov 24, 2020 at 11:29:15AM +0100, Petr Malat wrote:
> > > Both mmapped and compressed events can be split by the buffer boundary,
> > > it doesn't make sense to handle them differently.
> > I'm going to need more than this, if there's a problem
> > with current code please share more details, what's
> > broken and how it shows
> It's easy to trigger the problem - make a perf recording larger than
> MMAP_SIZE (32MB on 32-bit platform) and run perf report on it. There
> is a small chance recorded events will be aligned on the 32 MB
> boundary and in that case just repeat the test.
> 
> The problem was introduced by "perf session: Avoid infinite loop when
> seeing invalid header.size", which instead of aborting the execution
> when there is a truncated event at the end of the file just terminated
> execution whenever there is a split event. Later then the problem has
> been noticed for compressed events and fixed by "perf session: Fix
> decompression of PERF_RECORD_COMPRESSED records" by effectively
> reverting "perf session: Avoid infinite loop when seeing invalid
> header.size" for compressed events, which left uncompressed events
> broken.
> 
> I think the best is to revert these 2 changes and fix the original
> problem by aborting when there is no actual shift during remapping - as
> long as we shift, it's clear we must approach the end of the file so
> such an algorithm can't loop forever.
> BR,
>   Petr