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Subject[RFC/PATCH 0/15] Pass MAP_FIXED down to get_unmapped_area
!!! This is a first cut, and there are still cleanups to be done in various
areas touched by that code. I also haven't done descriptions yet for the
individual patches.

The current get_unmapped_area code calls the f_ops->get_unmapped_area or
the arch one (via the mm) only when MAP_FIXED is not passed. That makes
it impossible for archs to impose proper constraints on regions of the
virtual address space. To work around that, get_unmapped_area() then
calls some hugetlbfs specific hacks.

This cause several problems, among others:

- It makes it impossible for a driver or filesystem to do the same thing
that hugetlbfs does (for example, to allow a driver to use larger page
sizes to map external hardware) if that requires applying a constraint
on the addresses (constraining that mapping in certain regions and other
mappings out of those regions).

- Some archs like arm, mips, sparc, sparc64, sh and sh64 already want
MAP_FIXED to be passed down in order to deal with aliasing issues.
The code is there to handle it... but is never called.

This serie of patches moves the logic to handle MAP_FIXED down to the
various arch/driver get_unmapped_area() implementations, and then changes
the generic code to always call them. The hugetlbfs hacks then disappear
from the generic code.

Since I need to do some special 64K pages mappings for SPEs on cell, I need
to work around the first problem at least. I have further patches thus
implementing a "slices" layer that handles multiple page sizes through
slices of the address space for use by hugetlbfs, the SPE code, and possibly
others, but it requires that serie of patches first/

There is still a potential (but not practical) issue due to the fact that
filesystems/drivers implemeting g_u_a will effectively bypass all arch
checks. This is not an issue in practice as the only users of those are
actually doing so are doing it using arch hooks in the first place.

There is also a problem with mremap that will completely bypass all arch
checks. I'll try to address that separately mostly by making it not work
when the vma has a file whose f_ops has a get_unmapped_area callback,
and by making it use is_hugepage_only_range() before expanding into a
new area.

Also, I want to turn is_hugepage_only_range() into a more generic
is_normal_page_range() as that's really what it will end up meaning
when used in stack grow, brk grow and mremap.

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