gh-141770: Annotate anonymous mmap usage in debug builds only

[corona10]

• Issue: Name arenas allocated by mmap in _PyMem_ArenaAlloc for debugging #141770

[corona10]

Annotation

Without annotation

I verified that this annotation actually works, and I believe it will be very helpful for debugging when needed.

• Ruby: Annotate anonymous mmap ruby/ruby#12119
• Go: https://go.dev/doc/go1.25#vma-names-on-linux

[corona10]

cc @picnixz

[corona10]

FYI, Fedora does not support yet even if the kernel version itself already support it.
See: https://bugzilla.redhat.com/show_bug.cgi?id=2302746

cc @vstinner @encukou

[SpecLad]

FWIW, it would be useful to have an option to enable this in release builds too.

[corona10]

@SpecLad Thanks for the feedback, I will create a separate PR :)

[corona10]

@SpecLad Just out of curiosity, passing the build flag through ./configure --enable-annotate-mmap would be fine to you?
I would like to follow Ruby's approach but disable this flag by default unless many disto actually want this flag.

[SpecLad]

To be clear, by "option" I really mean a runtime option. 🙂 It would be pretty annoying to have to rebuild Python just for this. If you compare with other implementations, none of them require a compile-time option:

• C (glibc) - off by default, can be enabled with environment variable;
• Go - on by default, can be disabled with environment variable;
• Ruby - on permanently (unless I missed something).

TBH, I don't see why Python shouldn't also permanently enable this, but I would settle for "off by default, enabled with environment variable or -X option".

[picnixz]

It's a bit annoying that this would only be supported on Linux. Do we have an option that isn't supported everywhere?

[corona10]

@SpecLad Let’s move this forward conservatively for 3.15, and we can consider making it the default in the future. A runtime check would introduce an overall performance regression.
I still prefer the Ruby approach, since Ruby is also an interpreter like Python. But I do think enabling this by default later on is the right direction.

If some distros want to use this feature, they will enable it through a build flag, and it always happens.

[corona10]

It's a bit annoying that this would only be supported on Linux. Do we have an option that isn't supported everywhere?

Looks not and I belive that this is off-topic. :)

[picnixz]

My point was: I prefer not having an -X option that is platform dependent and rather have an environment variable (we have PYTHONCASEOK actually as a Windows-only environment variable).