linux-kernel - Re: [PATCH 01/20] perf nsinfo: fix refcounting

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Date:   Thu, 6 Jan 2022 17:34:08 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Krister Johansen <kjlx@...pleofstupid.com>
Cc:     Riccardo Mancini <rickyman7@...il.com>,
        Ian Rogers <irogers@...gle.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 01/20] perf nsinfo: fix refcounting

Em Thu, Jan 06, 2022 at 05:27:43PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Jan 04, 2022 at 10:19:42PM -0800, Krister Johansen escreveu:
> > On Thu, Jul 15, 2021 at 04:20:24PM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Thu, Jul 15, 2021 at 04:02:16PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > Em Thu, Jul 15, 2021 at 06:07:06PM +0200, Riccardo Mancini escreveu:
> > > > > ASan reports a memory leak of nsinfo during the execution of the perf
> > > > > test "31: Lookup mmap thread".
> > > > > The leak is caused by a refcounted variable being replaced without
> > > > > dropping the refcount.
> > > 
> > > > > This patch makes sure that the refcnt of nsinfo is decreased whenever
> > > > > a refcounted variable is replaced with a new value.
> > > 
> > > > So, there are multiple fixes in just one patch, I'll split it into
> > > > three, no need to resend.
> > > 
> > > > I'll try and check if finding Fixes: for the three is easy, that way
> > > > stable@...r.kernel.org will figure out which of the supported releases
> > > > need each of them.
> > > 
> > > The second is at the end of this message, and looking at it I think
> > > there is a problem with where this nsinfo reference is stored, it should
> > > be in the 'struct map', that is per-thread, not in the 'struct dso',
> > > that may be shared by multiple 'struct map's in different cgroups, so we
> > > can't set dso->nsinfo in map__new(), right? Kristen?
> > 
> > Apologies for the long delayed reply, and for the hassle that this has
> > caused.
> > 
> > You're right, Arnaldo, that namespace mapping could differ on a per
> > thread basis.  If the dso objects are intended to be read by multiple
> > threads with different views of the target process's namespaces.  With
> > containers, though, there might be many different versions of the same
> > object with the same filename, and the only ways we can disambiguate are
> > through the build-ids or by ensuring that the dso refers to a single
> > pid and mount namespace.  I may have erred in assuming that all of
> > perf's threads would be able to access the target namespace via the same
> > identifiers.  Is that what's going on here?
> 
> yeah, I think using build-ids, when available is the way to go, and
> since we can almost take it for granted (haven't checked) that it will
> be present, as I think gcc/clang adds them by default, seems the way to
> go.
> 
> That or somehow have the DSO list to be perf namespace.

To be by namespace, sorry.

But having it by build-id seems the more sensible way to go, so that we
can avoid having multiple instances of the same DSO in multiple
namespaces, which should be common.

I.e. continue to have a perf 'struct machine' DSO list instead of moving
to having the namespace hierarchy represented under 'struct machine'
since we have a way to uniquify DSOs, the build id.

One way or the other, nsinfo shouldn't be stored in DSOs, its a map
property, which in turn is a thread property.

I.e. please consider thinking about moving it to 'struct map', would you
try it, please?

- Arnaldo
 
> - Arnaldo
>  
> > -K
> > 
> > 
> > > commit 280f3933661f9ca9563ed04ce82313fdbcbb8f75
> > > Author: Riccardo Mancini <rickyman7@...il.com>
> > > Date:   Thu Jul 15 18:07:06 2021 +0200
> > > 
> > >     perf map: Fix dso->nsinfo refcounting
> > >     
> > >     ASan reports a memory leak of nsinfo during the execution of
> > >     
> > >       # perf test "31: Lookup mmap thread"
> > >     
> > >     The leak is caused by a refcounted variable being replaced without
> > >     dropping the refcount.
> > >     
> > >     This patch makes sure that the refcnt of nsinfo is decreased whenever a
> > >     refcounted variable is replaced with a new value.
> > >     
> > >     Signed-off-by: Riccardo Mancini <rickyman7@...il.com>
> > >     Fixes: bf2e710b3cb8445c ("perf maps: Lookup maps in both intitial mountns and inner mountns.")
> > >     Cc: Ian Rogers <irogers@...gle.com>
> > >     Cc: Jiri Olsa <jolsa@...hat.com>
> > >     Cc: Krister Johansen <kjlx@...pleofstupid.com>
> > >     Cc: Mark Rutland <mark.rutland@....com>
> > >     Cc: Namhyung Kim <namhyung@...nel.org>
> > >     Cc: Peter Zijlstra <peterz@...radead.org>
> > >     Link: http://lore.kernel.org/lkml/55223bc8821b34ccb01f92ef1401c02b6a32e61f.1626343282.git.rickyman7@gmail.com
> > >     Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> > > 
> > > diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
> > > index 8af693d9678cefe0..72e7f3616157ead4 100644
> > > --- a/tools/perf/util/map.c
> > > +++ b/tools/perf/util/map.c
> > > @@ -192,6 +192,8 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
> > >  			if (!(prot & PROT_EXEC))
> > >  				dso__set_loaded(dso);
> > >  		}
> > > +
> > > +		nsinfo__put(dso->nsinfo);
> > >  		dso->nsinfo = nsi;
> > >  
> > >  		if (build_id__is_defined(bid))
> > > 
> 
> -- 
> 
> - Arnaldo

-- 

- Arnaldo