MIPS: Make TASK_SIZE reflect proper size for both 32 and 64 bit processes.

The TASK_SIZE macro should reflect the size of a user process virtual
address space.  Previously for 64-bit kernels, this was not the case.
The immediate cause of pain was in
hugetlbfs/inode.c:hugetlb_get_unmapped_area() where 32-bit processes
trying to mmap a huge page would be served a page with an address
outside of the 32-bit address range.  But there are other uses of
TASK_SIZE in the kernel as well that would like an accurate value.

The new definition is nice because it now makes TASK_SIZE and
TASK_SIZE_OF() yield the same value for any given process.

For 32-bit kernels there should be no change, although I did factor
out some code in asm/processor.h that became identical for the 32-bit and
64-bit cases.

__UA_LIMIT is now set to ~((1 << SEGBITS) - 1) for 64-bit kernels.
This should eliminate the possibility of getting a
AddressErrorException in the kernel for addresses that pass the
access_ok() test.

With the patch applied, I can still run o32, n32 and n64 processes,
and have an o32 shell fork/exec both n32 and n64 processes.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1701/

arch/mips/kernel/cpu-probe.c

@@ -25,6 +25,8 @@
 #include <asm/system.h>
 #include <asm/watch.h>
 #include <asm/spram.h>
+#include <asm/uaccess.h>
+
 /*
  * Not all of the MIPS CPUs have the "wait" instruction available. Moreover,
  * the implementation of the "wait" feature differs between CPU families. This
@@ -987,6 +989,12 @@ static inline void cpu_probe_ingenic(struct cpuinfo_mips *c, unsigned int cpu)
 	}
 }
 
+#ifdef CONFIG_64BIT
+/* For use by uaccess.h */
+u64 __ua_limit;
+EXPORT_SYMBOL(__ua_limit);
+#endif
+
 const char *__cpu_name[NR_CPUS];
 const char *__elf_platform;
 
@@ -1064,6 +1072,11 @@ __cpuinit void cpu_probe(void)
 		c->srsets = 1;
 
 	cpu_probe_vmbits(c);
+
+#ifdef CONFIG_64BIT
+	if (cpu == 0)
+		__ua_limit = ~((1ull << cpu_vmbits) - 1);
+#endif
 }
 
 __cpuinit void cpu_report(void)