Bug #17652: GC compaction crash on mprotect

• Status: Closed
• Assignee: -
• Target version: -
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[peterzhu2118 (Peter Zhu)]

GitHub PR: https://github.com/ruby/ruby/pull/4221 https://github.com/ruby/ruby/pull/4227

Issue

GC compaction will crash on some pages due to a failing mprotect call. According to the mprotect documentation:

POSIX says that the behavior of mprotect() is unspecified if it is applied to a region of memory that was not obtained via mmap(2).

This causes mprotect to sometimes fail.

Reproduction

GC.auto_compact = true

times = 20_000_000
arr = Array.new(times)
times.times do |i|
  arr[i] = "#{i}"
end

arr = Array.new(1_000_000, 42)
GC.start

puts "ok"

Expected output

Program runs and prints "ok".

Actual output

Crashes on Ubuntu 18.04. See crash.log for the crash log (note that the log file has been truncated because it's too big).

[peterzhu2118 (Peter Zhu)]

• Description updated (diff)

[peterzhu2118 (Peter Zhu)]

• Description updated (diff)

[peterzhu2118 (Peter Zhu)]

• Description updated (diff)

[xtkoba (Tee KOBAYASHI)]

Changes made for this issue seem to break the Cygwin builds (for both i686 and x86_64). A backtrace for miniruby (without any options) from GDB:

Thread 1 received signal SIGSEGV, Segmentation fault.
heap_page_allocate (objspace=0x800053110) at ../gc.c:1883
1883        page_body->header.page = page;
(gdb) bt
#0  heap_page_allocate (objspace=0x800053110) at ../gc.c:1883
#1  heap_page_create (objspace=0x800053110) at ../gc.c:1923
#2  heap_assign_page (objspace=objspace@entry=0x800053110, heap=heap@entry=0x800053138) at ../gc.c:1948
#3  0x0000000100467ef1 in heap_add_pages (objspace=objspace@entry=0x800053110, heap=heap@entry=0x800053138, add=24) at ../gc.c:1961
#4  0x00000001004681be in Init_heap () at ../gc.c:3190
#5  0x000000010045653b in ruby_setup () at ../eval.c:87
#6  0x0000000000000000 in ?? ()
(gdb) p page_body
$1 = (struct heap_page_body *) 0x6fffffee4000
(gdb) p *page_body
Cannot access memory at address 0x6fffffee4000
(gdb)

I have not yet started to investigate what is wrong.

[xtkoba (Tee KOBAYASHI)]

• File ruby-cygwin-rb_aligned_malloc.patch ruby-cygwin-rb_aligned_malloc.patch added

It seems that the addr argument for Cygwin's munmap(2) must be aligned to 64KB boundaries. A patch is attached to conform to this.

[peterzhu2118 (Peter Zhu)]

Thanks for reporting the issue in cygwin. I've proposed a patch that should solve the issue.

[peterzhu2118 (Peter Zhu)]

• Status changed from Open to Closed