[ruby-core:116188] [Ruby master Bug#20169] `GC.compact` can raises `EFAULT` on IO

Issue #20169 has been updated by kjtsanaktsidis (KJ Tsanaktsidis).


Well I did a bit more thinking about this.

Firstly, I had a very unproductive morning trying to see if Mach exceptions on MacOS could catch invalid accesses in system calls, the way that `userfaultfd` can on Linux. Short answer: no.

The second insight I had, though, is that if these objects are on the machine stack, then they actually should be pinned anyway. If you have a C extension and you take a pointer to some internal part of an object, it's already a requirement that you ensure the Ruby value gets spilled to the stack - i.e. you need to do something like this.

```
VALUE str = rb_sprintf("i am a cool string");
write(fd, RSTRING_PTR(str), RSTRING_LEN(str));
RB_GC_GUARD(str); // spill string to stack; NOT OPTIONAL!
```

Mabye we could change the GC compaction algorithm to not move _any_ objects on a page (and hence skip protecting the page) if any objects in the page are live on the machine stack? That _would_ substantially lessen the effectiveness of GC compaction I suppose, but we could maybe get that effectiveness back if userfaultfd is available?


----------------------------------------
Bug #20169: `GC.compact` can raises `EFAULT` on IO
https://bugs.ruby-lang.org/issues/20169#change-106203

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
1. `GC.compact` introduces read barriers to detect read accesses to the pages.
2. I/O operations release GVL to pass the control while their execution, and another thread can call `GC.compact` (or auto compact feature I guess, but not checked yet).
3. Call `write(ptr)` can return `EFAULT` when `GC.compact` is running because `ptr` can point read-barrier protected pages (embed strings).

Reproducible steps:


Apply the following patch to increase possibility:

```patch
diff --git a/io.c b/io.c
index f6cd2c1a56..83d67ba2dc 100644
--- a/io.c
+++ b/io.c
@@ -1212,8 +1212,12 @@ internal_write_func(void *ptr)
         }
     }

+    int cnt = 0;
   retry:
-    do_write_retry(write(iis->fd, iis->buf, iis->capa));
+    for (; cnt < 1000; cnt++) {
+        do_write_retry(write(iis->fd, iis->buf, iis->capa));
+        if (result <= 0) break;
+    }

     if (result < 0 && !iis->nonblock) {
         int e = errno;
```

Run the following code:

```ruby
t1 = Thread.new{ 10_000.times.map{"#{_1}"}; GC.compact while true }
t2 = Thread.new{
  i=0
  $stdout.write "<#{i+=1}>" while true
}
t2.join
```

and 

```
$ make run
(snip)
4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4>#<Thread:0x00007fa61b4dd758 ../../src/trunk/test.rb:3 run> terminated with exception (report_on_exception is true):
../../src/trunk/test.rb:5:in `write': Bad address @ io_write - <STDOUT> (Errno::EFAULT)
        from ../../src/trunk/test.rb:5:in `block in <main>'
../../src/trunk/test.rb:5:in `write': Bad address @ io_write - <STDOUT> (Errno::EFAULT)
        from ../../src/trunk/test.rb:5:in `block in <main>'
make: *** [uncommon.mk:1383: run] Error 1
```

I think this is why we get `EFAULT` on CI. To increase possibilities running many busy processes (`ruby -e 'loop{}'` for example) will help (and on CI environment there are such busy processes accidentally).



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