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    SubjectRe: [PATCH] Inconsistent mmap()/mremap() flags
    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().

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      Last update: 2007-10-02 09:15    [W:0.020 / U:68.140 seconds]
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