Re: [RFC] Preventing overlap between arch mman.h and asm-generic/mman.h

On 11/21/2012 3:42 PM, Carlos O'Donell wrote:

David,

Thanks for the great UAPI work. While working on glibc mman.h sync I had
the chance to look more closely at some of the UAPI headers.

Currently (3.7.0-rc6) sparc, powerpc, and tile are including mman-common.h,
but nothing in mman-common.h helps prevent the accidental definition of an
overlapping new constant value.

For example in mman.h powerpc defines:
 #define MAP_NORESERVE  0x40            /* don't reserve swap pages */
 #define MAP_LOCKED     0x80

If someone were to add a new MAP_* value in mman-common.h at 0x40 it
would conflict with the existing powerpc value. The powerpc maintainers
might not notice, and that's a bug.

I notice that someone is trying to avoid this kind of bug, because for
example the new MADV_HWPOISON was chosen as 100, which is higher than
any other MADV_* value for any architecture.

Rather than avoid the problem by spot-checking, would it be acceptable
to merge in some private macros that block out exactly which constants
are already in use as existing values for arches using UAPI headers?

Doing it this way would allow for the merged arches to more safely
include the asm-generic version of mman.h and start benefiting from new
constants without the usual maintenance cost (not to mention everyone
would start in lock-step going forward). Not to mention it simplifies
some of the glibc maintenance.

This proposal looks reasonable to me and the proposed code seems OK for
tile, at least.  Thanks!

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com