Re: [Qemu-devel] linux-user mmap bug

[Richard Henderson <rth@twiddle.net>]

On 05/24/2010 07:57 AM, Edgar E. Iglesias wrote:
> I took a look at the code again and I dont really understand how the
> particular case when we get a high address from the kernel while
> mmap_min_addr is busy case is supposed to work :/
> In fact, for CRIS it never works on my host.

Indeed, there are many cases for which it doesn't work for the Alpha
target either.

> I changed it locally to keep scanning after a wrap until we succeed to
> allocate a chunk or rewrap (SLOW) but at least I can run dynamically
> linked CRIS programs again.

Yep.  My hack had been similar, except that I used the PageDesc tree
to help speed things up.  But PageDesc is hardly an ideal data structure
in which to search, since it quickly devolves into a linear search of
the address space.

Probably the easiest real fix is to re-read /proc/self/maps each time
the mmap_next_start guess fails and the kernel's returned address is
out of range.

Another is using the MMAP_32BIT flag on x86-64 host whenever a 31-bit
address is appropriate for the guest.  E.g. mips32, where architecturally
the high half of the address space is reserved for kernel mode.

See 
  http://www.mail-archive.com/qemu-devel@nongnu.org/msg28924.html
for more ideas on the subject.



r~