Why should I include highmem.h?

[Timur Tabi <timur.tabi@xxxxxxxxxxx>]

I have a driver that compiles with several different 2.4 and 2.6 
kernels.  In one file, if I include <linux/highmem.h>, I sometimes get 
strange compilation errors, like this one when I compile with Red Hat 
Enterprise 3:

In file included from 
/usr/ammasso/src/rhel3/2.4.21-4smp-32/include/asm/desc.h:9,
                 from 
/usr/ammasso/src/rhel3/2.4.21-4smp-32/include/asm/pgalloc.h:7,
                 from 
/usr/ammasso/src/rhel3/2.4.21-4smp-32/include/linux/highmem.h:5,
                 from devccil_mm.c:17:
/usr/ammasso/src/rhel3/2.4.21-4smp-32/include/asm/mmu.h:18: field `sem' 
has incomplete type
/usr/ammasso/src/rhel3/2.4.21-4smp-32/include/asm/mmu.h:22: confused by 
earlier errors, bailing out

If I don't include highmem.h, everything compiles correctly.  So what is 
the value of highmem.h?  What do I get by including it, and if I do need 
it, how do I include it without generating these errors?

--
Timur Tabi
Staff Software Engineer
timur.tabi@xxxxxxxxxxx

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