Re: [PATCH] Inconsistent mmap()/mremap() flags

Thayne Harbaugh
2007-10-02T07:13:26Z

On Tue, 2007-10-02 at 07:15 +0200, Andi Kleen wrote:
> On Mon, Oct 01, 2007 at 08:57:10PM -0600, Thayne Harbaugh wrote:

> For mmap you can emulate it by passing a low hint != 0 (e.g. getpagesize())
> in address but without MAP_FIXED and checking if the result is not beyond
> your range.

Cool. That's a much better solution for multiple reasons - like you
mention, MAP_32BIT is only 2GB as well as it's only available on x86_64.

> > > Given for mremap() it is not that easy because there is no "hint" argument
> > > without MREMAP_FIXED; but unless someone really needs it i would prefer
> > > to not propagate the hack. If it's really needed it's probably better
> > > to implement a start search hint for mremap()
> >
> > It came up for user-mode Qemu for the case of emulating 32bit archs on
> > x86_64 using mmap. At the moment it calls mmap with MAP_32BIT and then
>
> That would limit the 32bit architectures to 2GB; but their real limit
> is 4GB. Losing half of the address space definitely would make users unhappy
> (e.g. at least normal Linux kernels wouldn't run at all)

Keeping a kernel happy isn't necessary since it's user-space emulation
rather than full emulation. It is, however, useful to have 4GB rather
than 2GB.

> Does qemu actually need mremap() ? It would surprise me because
> a lot of other OS don't implement it.

Qemu has two modes: full hardware emulation and user-mode emulation.
User-mode emulation translates the user-mode code and then remaps the
system calls directly into the native kernel (that way all the kernel
and all the I/O runs natively and faster). As far as mremap(), I'm
trying to get a 32bit arm mremap() emulated syscall mapped onto a 64bit
x86_64 mremap().