mmaping a fixed address fails on ARM

[Bryan Wu]

On Thu, Jan 13, 2011 at 5:44 PM, Mikael Pettersson <mikpe at it.uu.se> wrote:
> Bryan Wu writes:
>  > Let me add Andre here
>  >
>  > Thanks,
>  > -Bryan
>  >
>  > On Thu, Jan 13, 2011 at 5:58 AM, Bryan Wu <bryan.wu at canonical.com> wrote:
>  > > Hi Russell,
>  > >
>  > > Andre posted a mmap testcase [1] and a bug report [2] for Ubuntu
>  > > kernel on OMAP4 system, since he is porting some applications from x86
>  > > to ARM. He testcase works fine on x86, but always fail on ARM. I've
>  > > tested it on OMAP3/OMAP4/i.MX51.
>  > >
>  > > Basically what he want to do is get a mapping on a specific virtual
>  > > address. But AFAIK, mmap doesn't make sure we can get the mapping
>  > > address as we want. On x86, it works fine. So if this fails, the
>  > > application can't run on ARM.
>  > >
>  > > The implementation of arch_get_unmapped_area() is different from ARM
>  > > and x86, that might makes mmap behavior different between ARM and x86.
>  > > Andre said 2.6.28-versatile. I'm not sure whether it is a regression.
>  > >
>  > > [1]: http://launchpadlibrarian.net/61588086/main.c
>  > > [2]: https://bugs.launchpad.net/ubuntu/+source/linux-ti-omap4/+bug/697004
>
> mmap() with a non-NULL hint referring to a currently unmapped area,
> but without MAP_FIXED, isn't even remotely portable or reliable
> (I've tried it myself in the context of JITs).  It may work on
> Linux/x86, but that doesn't allow you to assume it should work
> on other OS/CPU combinations (and I know of several where it
> emphatically doesn't work).
>

Exactly. I also think the mmap() behavior is correct in this test
scenario. The address location was mapped by some LD libraries, so
mmaping those address won't get the address we expected.

> If you want a fixed address then use MAP_FIXED.  If you don't want
> MAP_FIXED to clobber an existing mapping then you have to keep track
> of existing mappings yourself and avoid them.
>
> What is that "some application"?
>

I guess it is 'wine', don't be surprise. -:)

Thanks,
-- 
Bryan Wu <bryan.wu at canonical.com>
Kernel Developer    +86.138-1617-6545 Mobile
Ubuntu Kernel Team
Canonical Ltd.      www.canonical.com
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