Re: Preview: portable dumper

[Daniel Colascione]

On 02/15/2018 05:56 PM, Ken Brown wrote:
> On 2/15/2018 6:36 PM, Daniel Colascione wrote:
> > On Feb 15, 2018 3:31 PM, Ken Brown <address@hidden> wrote:
> >
> >     On 2/13/2018 11:37 AM, Eli Zaretskii wrote:
> >      >> From: Daniel Colascione <address@hidden>
> >      >> Date: Mon, 12 Feb 2018 12:18:36 -0800
> >      >>
> >      >> I've pushed the portable dumper to the pdumper Savannah 
> > branch. It
> >      >> should support a fully PIC Emacs.
> >      >
> >      > Thanks.  I'd urge people to try this branch and report any issues
> >     they
> >      > see.
> >
> >     I just tried to build on 64-bit Cygwin, and the build fails as 
> > follows:
> >
> >     [...]
> >     Dumping under the name bootstrap-emacs.pdmp
> >     dumping fingerprint:
> >     923050a9f611ad7ead76eea704308e4d05f152601a9134cf8d1b5ff3e0e1a986
> >     Dump complete
> >     Byte counts: header=80 hot=13187392 discardable=119424 cold=9086640
> >     Reloc counts: hot=919268 discardable=5790
> >     make -C ../lisp compile-first EMACS="../src/bootstrap-emacs.exe"
> >     make[2]: Entering directory
> >     '/home/kbrown/src/emacs/x86_64-pdumper/lisp'
> >         ELC      ../../pdumper/lisp/emacs-lisp/macroexp.elc
> >         ELC      ../../pdumper/lisp/emacs-lisp/cconv.elc
> >         ELC      ../../pdumper/lisp/emacs-lisp/byte-opt.elc
> >         ELC      ../../pdumper/lisp/emacs-lisp/bytecomp.elc
> >     emacs: could not load dump file "../src/bootstrap-emacs.pdmp": out
> >     of memory
> >
> >     There's probably some obvious explanation, but I don't see it at the
> >     moment.
> >
> >
> > I'm not entirely surprised to see Cygwin fall over here. We could just 
> > use the Windows memory mapping functions directly, but I'd prefer to 
> > stick with the POSIX API if we can make it work.
>
> I agree.
>
> > Any idea where in the mmap sequence we fail? 
>
> I haven't looked at the code yet, so I don't understand the question. If 
> you give me some guidance, I'll try to investigate.

Thanks. I think the trouble must be in dump_mmap_contiguous. We report 
all errors from this function as "out of memory". dump_mmap_contiguous 
takes an array of mapping descriptors (think ELF segments or something) 
and maps them all into a single contiguous region of virtual memory. On 
POSIX systems, we reserve a chunk of address space with a big PROT_NONE 
anonymous mapping, then carve it up into the separate mappings that we 
really want. Windows doesn't support atomic mmap replacement, so Cygwin 
has to emulate it, and I wouldn't be surprised if something's going 
wrong in this emulation.

That you're able to compile a few elisp files before bootstrap starts 
failing makes me wonder whether we're dealing with Cygwin's perennial 
problem with address space collisions and some kind of dynamic-base DLL.