riscv: Improve arch_get_mmap_end() macro

This macro caused me some confusion, which took some reviewer's time to
make it clear, so I propose adding a short comment in code to avoid
confusion in the future.

Also, added some improvements to the macro, such as removing the
assumption of VA_USER_SV57 being the largest address space.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20240103160024.70305-3-leobras@redhat.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: Leonardo Bras

arch/riscv/include/asm/processor.h

@@ -18,15 +18,21 @@
 #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
 #define STACK_TOP_MAX		TASK_SIZE_64
 
+/*
+ * addr is a hint to the maximum userspace address that mmap should provide, so
+ * this macro needs to return the largest address space available so that
+ * mmap_end < addr, being mmap_end the top of that address space.
+ * See Documentation/arch/riscv/vm-layout.rst for more details.
+ */
 #define arch_get_mmap_end(addr, len, flags)			\
 ({								\
 	unsigned long mmap_end;					\
 	typeof(addr) _addr = (addr);				\
 	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
 		mmap_end = STACK_TOP_MAX;			\
-	else if ((_addr) >= VA_USER_SV57)			\
-		mmap_end = STACK_TOP_MAX;			\
-	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) \
+	else if (((_addr) >= VA_USER_SV57) && (VA_BITS >= VA_BITS_SV57)) \
+		mmap_end = VA_USER_SV57;			\
+	else if (((_addr) >= VA_USER_SV48) && (VA_BITS >= VA_BITS_SV48)) \
 		mmap_end = VA_USER_SV48;			\
 	else							\
 		mmap_end = VA_USER_SV39;			\