Re: [PATCH v3 1/3] riscv: mm: Use hint address in mmap if available

[Yangyu Chen <cyy@cyyself.name>]

On Tue, 2024-01-30 at 17:07 -0800, Charlie Jenkins wrote:
> On riscv it is guaranteed that the address returned by mmap is less
> than
> the hint address. Allow mmap to return an address all the way up to
> addr, if provided, rather than just up to the lower address space.
> 
> This provides a performance benefit as well, allowing mmap to exit
> after
> checking that the address is in range rather than searching for a
> valid
> address.
> 
> It is possible to provide an address that uses at most the same
> number
> of bits, however it is significantly more computationally expensive
> to
> provide that number rather than setting the max to be the hint
> address.
> There is the instruction clz/clzw in Zbb that returns the highest set
> bit
> which could be used to performantly implement this, but it would
> still
> be slower than the current implementation. At worst case, half of the
> address would not be able to be allocated when a hint address is
> provided.
> 
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> ---
>  arch/riscv/include/asm/processor.h | 27 +++++++++++----------------
>  1 file changed, 11 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h
> b/arch/riscv/include/asm/processor.h
> index f19f861cda54..8ece7a8f0e18 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -14,22 +14,16 @@
>  
>  #include <asm/ptrace.h>
>  
> -#ifdef CONFIG_64BIT
> -#define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
> -#define STACK_TOP_MAX		TASK_SIZE_64
> -
>  #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())) \
> +	if ((_addr) == 0 ||					\
> +	    (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) ||	\
> +	    ((_addr + len) > BIT(VA_BITS -
> 1)))			\
>  		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)) \
> -		mmap_end = VA_USER_SV48;			\
>  	else							\
> -		mmap_end = VA_USER_SV39;			\
> +		mmap_end = (_addr + len);			\
>  	mmap_end;						\
>  })
>  
> @@ -39,17 +33,18 @@
>  	typeof(addr) _addr = (addr);				\
>  	typeof(base) _base = (base);				\
>  	unsigned long rnd_gap = DEFAULT_MAP_WINDOW - (_base);	\
> -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) &&
> is_compat_task())) \
> +	if ((_addr) == 0 ||					\
> +	    (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) ||	\
> +	    ((_addr + len) > BIT(VA_BITS -
> 1)))			\
>  		mmap_base = (_base);				\
> -	else if (((_addr) >= VA_USER_SV57) && (VA_BITS >=
> VA_BITS_SV57)) \
> -		mmap_base = VA_USER_SV57 - rnd_gap;		\
> -	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >=
> VA_BITS_SV48)) \
> -		mmap_base = VA_USER_SV48 - rnd_gap;		\
>  	else							\
> -		mmap_base = VA_USER_SV39 - rnd_gap;		\
> +		mmap_base = (_addr + len) - rnd_gap;		\
>  	mmap_base;						\
>  })
>  
> +#ifdef CONFIG_64BIT
> +#define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
> +#define STACK_TOP_MAX		TASK_SIZE_64
>  #else
>  #define DEFAULT_MAP_WINDOW	TASK_SIZE
>  #define STACK_TOP_MAX		TASK_SIZE
> 

I have carefully tested your patch on qemu with sv57. A bug that needs
to be solved is that mmap with the same hint address without MAP_FIXED
set will fail the second time.

Userspace code to reproduce the bug:

#include <sys/mman.h>
#include <stdio.h>
#include <stdint.h>

void test(char *addr) {
    char *res = mmap(addr, 4096, PROT_READ | PROT_WRITE, MAP_ANONYMOUS
| MAP_PRIVATE, -1, 0);
    printf("hint %p got %p.\n", addr, res);
}

int main (void) {
    test(1<<30);
    test(1<<30);
    test(1<<30);
    return 0;
}

output:

hint 0x40000000 got 0x40000000.
hint 0x40000000 got 0xffffffffffffffff.
hint 0x40000000 got 0xffffffffffffffff.

output on x86:

hint 0x40000000 got 0x40000000.
hint 0x40000000 got 0x7f9171363000.
hint 0x40000000 got 0x7f9171362000.

It may need to implement a special arch_get_unmapped_area and
arch_get_unmapped_area_topdown function.