[PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

[Arnd Bergmann]

On Fri, Sep 6, 2024, at 09:14, Guo Ren wrote:
> On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann <arnd at arndb.de> wrote:
>>
>> It's also unclear to me how we want this flag to interact with
>> the existing logic in arch_get_mmap_end(), which attempts to
>> limit the default mapping to a 47-bit address space already.
>
> To optimize RISC-V progress, I recommend:
>
> Step 1: Approve the patch.
> Step 2: Update Go and OpenJDK's RISC-V backend to utilize it.
> Step 3: Wait approximately several iterations for Go & OpenJDK
> Step 4: Remove the 47-bit constraint in arch_get_mmap_end()

I really want to first see a plausible explanation about why
RISC-V can't just implement this using a 47-bit DEFAULT_MAP_WINDOW
like all the other major architectures (x86, arm64, powerpc64),
e.g. something like the patch below (untested, probably slightly
wrong but show illustrate my point).

     Arnd

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 8702b8721a27..de9863be1efd 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -20,17 +20,8 @@
  * 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_compat_task() ||			\
-	    ((_addr + len) > BIT(VA_BITS - 1)))			\
-		mmap_end = STACK_TOP_MAX;			\
-	else							\
-		mmap_end = (_addr + len);			\
-	mmap_end;						\
-})
+#define arch_get_mmap_end(addr, len, flags) \
+		(((addr) > DEFAULT_MAP_WINDOW) ? TASK_SIZE : DEFAULT_MAP_WINDOW)
 
 #define arch_get_mmap_base(addr, base)				\
 ({								\
@@ -47,7 +38,7 @@
 })
 
 #ifdef CONFIG_64BIT
-#define DEFAULT_MAP_WINDOW     (UL(1) << (MMAP_VA_BITS - 1))
+#define DEFAULT_MAP_WINDOW     (is_compat_task() ? (UL(1) << (MMAP_VA_BITS - 1)) : TASK_SIZE_32)
 #define STACK_TOP_MAX          TASK_SIZE_64
 #else
 #define DEFAULT_MAP_WINDOW     TASK_SIZE