[ruby-core:116208] [Ruby master Bug#20184] Ruby segfaults on Fly.io with 256 MB RAM

["ioquatix (Samuel Williams) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20184 has been updated by ioquatix (Samuel Williams).


I was also investigating this issue.

On low-memory systems (< ~400MB RAM+Swap), `Init_default_shapes` will fail and cause Ruby to segfault later.

Even something as simple as ruby -e "puts 123" will fail.

The 2nd `mmap` in `shape.c` tries to allocate 384MB as a cache. This fails with `MAP_FAILED` which is stored into a `shape_cache` pointer and later this pointer is derefenced causing a segfault.

The current merged fix skips the cache if the `mmap` fails, but might I suggest we explore the following:

- Divide and conquer strategy for finding an allocation size that works (e.g. divide by half and try again a few times).
- Expose an environment variable to control the size, so that those on memory constrained systems can at least enable a smaller cache.


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Bug #20184: Ruby segfaults on Fly.io with 256 MB RAM
https://bugs.ruby-lang.org/issues/20184#change-106228

* Author: aalin (Andreas Alin)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-linux]
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED
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I've been running a Ruby 3.2 app on Fly.io with 256 MB RAM and it has been working fine.
When I upgraded to Ruby 3.3.0, any Ruby script would crash immediately with a segfault.

I haven't been able to reproduce it outside of fly.io.

I created a repository with a basic rack app which crashes on fly.io.
The readme includes the full output and a GDB backtrace.
https://github.com/aalin/ruby-3-3-0-fly-crash



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