[PATCH v2 2/2] riscv: Define TASK_SIZE_MAX for __access_ok()

[Samuel Holland]

TASK_SIZE_MAX should be set to a constant value, at least the largest
valid userspace address under any runtime configuration. This optimizes
the check in __access_ok(), which no longer needs to compute the runtime
value of TASK_SIZE. The check does not need to be exact, as long as it
accepts all valid userspace addresses and rejects all valid kernel
addresses; well-behaved programs will never fail the access_ok() check.

For RISC-V, which requires all virtual addresses to be sign extended,
the optimal choice is LONG_MAX because it simplifies the limit
comparison to a sign bit test.

This removes about half of the references to pgtable_l[45]_enabled.

Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

Changes in v2:
 - Set TASK_SIZE_MAX to LONG_MAX to optimize the comparison
 - Reword the commit message

 arch/riscv/include/asm/pgtable.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index f5cc8bcc7f8d..762a85551764 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -870,6 +870,7 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
  */
 #ifdef CONFIG_64BIT
 #define TASK_SIZE_64	(PGDIR_SIZE * PTRS_PER_PGD / 2)
+#define TASK_SIZE_MAX	LONG_MAX
 
 #ifdef CONFIG_COMPAT
 #define TASK_SIZE_32	(_AC(0x80000000, UL) - PAGE_SIZE)
-- 
2.43.1