[ruby-core:119566] [Ruby master Bug#20804] Stop reserving stack ahead-of-time in on Linux

["mame (Yusuke Endoh) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20804 has been updated by mame (Yusuke Endoh).


In short, you are saying that the issue is supposed to have been fixed since Linux 4.13, right? Then I don't see a problem.

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Bug #20804: Stop reserving stack ahead-of-time in on Linux
https://bugs.ruby-lang.org/issues/20804#change-110180

* Author: kjtsanaktsidis (KJ Tsanaktsidis)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
In Linux, the main thread generally only gets a small stack mapped in initially. As the application attempts to use more stack memory, the kernel will map in more stack pages.

In https://github.com/ruby/ruby/pull/822, we added some logic to force the kernel to eagerly map and fault in the entire stack by writing a fake array near the bottom. This was done in order to fix some cases where heap memory was unexpectedly being allocated in locations close to the stack, which then prevented the stack from growing.

I ran into this because this logic needed to be fixed for ASAN (https://github.com/ruby/ruby/pull/11921). However, I actually think we should delete `reserve_stack` entirely, which is the point of this issue.

Myself and @rianmcguire had a look at this today and we believe that the original problem was in fact a symptom of a kernel bug. The kernel bug (or at least, what we _think_ was the relevant bug) was fixed in 2017 (https://github.com/torvalds/linux/commit/c204d21f2232d875e36b8774c36ffd027dc1d606) On my machine today, under ruby 3.3.2 (2024-05-30 revision e5a195edf6) and kernel 6.10.12-200.fc40.x86_64 I can no longer reproduce the problem demonstrated by the repro script (https://gist.github.com/csfrancis/46e360d401609275246c).

```
kjtsanaktsidis@kjtsanaktsidis-laptop ~ % ./repro.rb
new minimum diff: 140730206072832 (2)
new minimum diff: 140725853872128 (4)
new minimum diff: 140723732975616 (10)
new minimum diff: 140719631585280 (14)
new minimum diff: 140719552581632 (69)
new minimum diff: 140719410409472 (159)
new minimum diff: 140719327940608 (1191)
new minimum diff: 140719326601216 (3111)
new minimum diff: 140719312199680 (6098)
```

Performing this kind of stack reservation actually causes _other_ problems - if RLIMIT_STACK is set to a high value, performing the eager mapping like this can fail for lack of real memory

```
kjtsanaktsidis@kjtsanaktsidis-laptop ~ % ulimit -s 1000000000
kjtsanaktsidis@kjtsanaktsidis-laptop ~ % ruby -e "puts 'hi'"
zsh: segmentation fault (core dumped)  ruby -e "puts 'hi'"
```

So, therefore, I believe the right thing to do is to just delete `reserve_stack`. Are there any objections to doing this?



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