mmaping a fixed address fails on ARM
bryan.wu at canonical.com
Thu Jan 13 17:31:31 EST 2011
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  and a bug report  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.
> > >
> > > : http://launchpadlibrarian.net/61588086/main.c
> > > : 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. -:)
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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