[m-rev.] for review: Enable munmap support in Boehm GC by default.

[Peter Wang]

On Fri, 24 Jul 2020 16:55:14 +1000 Julien Fischer <jfischer at opturion.com> wrote:
> 
> Hi Peter,
> 
> On Thu, 23 Jul 2020, Peter Wang wrote:
> 
> > On Wed, 22 Jul 2020 22:18:12 +1000 Peter Wang <novalazy at gmail.com> wrote:
> >> On Wed, 22 Jul 2020 21:54:49 +1000 Julien Fischer <jfischer at opturion.com> wrote:
> >>>
> >>>>
> >>>> Some testing on my machine suggests that we can try enabling munmap
> >>>> support by default:
> >
> > Unfortunately, I've found that by aggressively calling
> > GC_gcollect_and_unmap() I can sometimes hit an abort:
> >
> >    mmap(PROT_NONE) failed
> >
> > I'm going to revert this change until I can figure it out.
> 
> Is this likely to be something that moving to the 8.0 branch would
> address?

No. I discovered that when munmap support is enabled the number of
memory map areas it creates may exceed the value of vm.max_map_count
on Linux (other OSes may have similar limits). It's probably not a
requirement to call GC_gcollect_and_unmap() to hit the problem,
but it does make it more likely.

I've opened an issue here: https://github.com/ivmai/bdwgc/issues/324

Peter