Re: [PATCH v2 1/3] riscv: mm: Use hint address in mmap if available

[Yangyu Chen]

On Tue, 2024-01-30 at 11:04 -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@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/processor.h | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h
> b/arch/riscv/include/asm/processor.h
> index f19f861cda54..5d966ae81a58 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -22,14 +22,12 @@
>  ({								\
>  	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)) \
> -		mmap_end = VA_USER_SV48;			\
> +	if ((_addr) == 0 ||					\
> +	    (IS_ENABLED(CONFIG_COMPAT) && is_compat_task()) ||	\
> +	    ((_addr + len) > BIT(VA_BITS -
> 1)))			\
> +		mmap_end = STACK_TOP_MAX			\
>  	else							\
> -		mmap_end = VA_USER_SV39;			\
> +		mmap_end = (_addr + len);			\
>  	mmap_end;						\
>  })
>  
> @@ -39,14 +37,12 @@
>  	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;		\

Please mind that rnd_gap can be non-zero, in this case, the map will
fail. It will be better to let mmap_base = min((_addr + len), (base) +
TASK_SIZE - DEFAULT_MAP_WINDOW) .

>  	mmap_base;						\
>  })
>  
>