Re: [PATCH 13/24] perf tools: Allow mmap2 event to synthesize kernel image

[Jiri Olsa]

On Tue, Nov 17, 2020 at 09:44:37AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Nov 17, 2020 at 12:00:42PM +0100, Jiri Olsa escreveu:
> > Allow mmap2 event to synthesize kernel image,
> > so we can synthesize kernel build id data in
> > following changes.
> > 
> > It's enabled by new symbol_conf.buildid_mmap2
> > bool, which will be switched in following
> > changes.
> 
> Why make this an option? MMAP2 goes back years:
> 
> 13d7a2410fa637f45 (Stephane Eranian         2013-08-21 12:10:24 +0200  904)      * The MMAP2 records are an augmented version of MMAP, they add
> 13d7a2410fa637f45 (Stephane Eranian         2013-08-21 12:10:24 +0200  905)      * maj, min, ino numbers to be used to uniquely identify each mapping
> 
> Also we unconditionally generate MMAP2 events if the kernel supports it,
> from evsel__config():
> 
>   attr->mmap  = track;
>   attr->mmap2 = track && !perf_missing_features.mmap2;
> 
> So perhaps we should reuse that logic? I.e. use mmap2 if the kernel
> supports it?

mmap2 itself is not a problem, the problem is the new
bit (PERF_RECORD_MISC_MMAP_BUILD_ID) that says there's
build id in mmap2.. older perf tool won't understand
that and report will crash

jirka