[Gc] Allocating Executable Memory

Boehm, Hans hans.boehm at hp.com
Fri Jul 23 12:10:12 PDT 2010


I don't mind this either.  We've generally been moving static configuration options to runtime wherever the performance impact was minor.  We didn't get around to this one yet.

Hans

> -----Original Message-----
> From: gc-bounces at linux.hpl.hp.com [mailto:gc-bounces at linux.hpl.hp.com]
> On Behalf Of Ivan Maidanski
> Sent: Friday, July 23, 2010 11:40 AM
> To: Noah Lavine
> Cc: gc at linux.hpl.hp.com
> Subject: Re: [Gc] Allocating Executable Memory
> 
> Hello!
> 
> I like the idea.
> I don't mind adding this feature in this release (probably Hans
> wouldn't neither since the added coded is small and easily verifiable).
> 
> The API should be:
> - GC_set_pages_executable(int) // non-zero means executable
> - GC_get_pages_executable() // returns non-zero if it is allowed to
> execute code in allocated memory
> 
> NO_EXECUTE_PERMISSION controls the initial value only.
> 
> If you'd like to add it, I expect you'll provide the patch against the
> current CVS.
> 
> 
> Fri, 23 Jul 2010 13:28:17 -0400 Noah Lavine <noah.b.lavine at gmail.com>:
> 
> > Hello GC Developers,
> >
> > I am writing with a feature request for the next version of your
> library.
> >
> > I am working on adding JIT compilation support to GNU Guile (a Scheme
> > implementation), which uses this library for garbage collection. In
> > order to make GC work, we'll need to allocate executable memory.
> > However, I have discovered that allocating executable memory is a
> > build-time configuration option (in version 7.1, it appears to be set
> > in configure.ac, at lines 401 and 495). This gives Guile something of
> > a problem - currently we use whatever libgc is on a user's computer.
> > However, the only way to ensure that libgc will allocate executable
> > memory would be to build our own version of it, which would add space
> > to our executable and be redundant. We could also hack together our
> > own memory allocator only for executable memory, but we like using
> > your library. :-)
> >
> > Therefore, I'd like to request that the next version of the library
> > have allocating executable memory as a runtime configuration option
> > that can be set by programs. I don't think it would affect
> performance
> > very much - in fact, I can find only two uses of it in the entire
> > program (calls to mmap and mprotect in os_dep.c).
> >
> > I also have a question - is there some way to make an older version
> of
> > libgc manage executable memory anyway? For instance, if I mmap some
> > pages of executable memory, will libgc scan those for pointers even
> > though it didn't allocate them? Are there other ways to work around
> > this?
> 
> See GC_add_roots.
> 
> >
> > Thank you very much
> > Noah Lavine
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> > http://www.hpl.hp.com/hosted/linux/mail-archives/gc/
> 
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