Re: MPS codegen

[Eli Zaretskii <eliz@gnu.org>]

> From: Helmut Eller <eller.helmut@gmail.com>
> Cc: gerd.moellmann@gmail.com,  acorallo@gnu.org,  emacs-devel@gnu.org
> Date: Fri, 14 Jun 2024 21:26:37 +0200
> 
> On Fri, Jun 14 2024, Eli Zaretskii wrote:
> 
> > More generally, what exactly has been changed wrt loading from the
> > pdumper file since commit 219f7d5?
> 
> The file is first loaded (mostly) as before; then all objects from the
> hot section are copied to a MPS managed region.  Finally the hot section
> is discarded.

Can't we instead tell MPS not to move or manage any objects loaded
from the pdumper file?  That's basically what we do now on master: any
object loaded from the pdumper file is considered immutable.

> My hypothesis is that discard_dump is the problem.  The hot section
> probably contains something that isn't properly copied and
> dump_discard_mem seems to be a bit different on Windows.  Why it even
> works on Linux is a mystery to me :-).

The Windows code decommits the memory region and makes it
inaccessible.  By contrast, the posix_madvise method is AFAIU just an
advisory, and doesn't necessarily make the memory inaccessible.
Moreover, the Linux man page says:

       In glibc, this function is implemented using madvise(2).
       However, since glibc 2.6, POSIX_MADV_DONTNEED is treated as a no-
       op, because the corresponding madvise(2) value, MADV_DONTNEED,
       has destructive semantics.

So to do on Posix systems the same as we do on Windows, I think we
need to call madvise with MADV_DONTNEED, even if posix_madvise is
available, and also call mmap with PROT_NONE.  Do you agree?  If we do
that, does the GNU/Linux build still work, or does it also crash on
startup?