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

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Jan 31, 2024 at 03:15:16AM +0800, Yangyu Chen wrote:
> 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@rivosinc.com>
> > ---
> >  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) .

Why would an rnd_gap that is non-zero cause mmap to fail? mmap will fail
if rnd_gap is greater than (_addr + len). That is expected and should be
interpreted as no address was available in the requested address space.

mmap_base is used if the hint address is not available.

I do see that I fumbled the test cases in this patch so I will fix that
in a v3.

- Charlie

> 
> >  	mmap_base;						\
> >  })
> >  
> > 
>