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

[Charlie Jenkins]

On Mon, Jan 29, 2024 at 08:53:31PM -0500, Stefan O'Rear wrote:
> On Mon, Jan 29, 2024, at 7:37 PM, 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 | 21 ++++++++-------------
> >  1 file changed, 8 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/processor.h 
> > b/arch/riscv/include/asm/processor.h
> > index f19f861cda54..f3ea5166e3b2 100644
> > --- a/arch/riscv/include/asm/processor.h
> > +++ b/arch/riscv/include/asm/processor.h
> > @@ -22,14 +22,11 @@
> >  ({								\
> >  	unsigned long mmap_end;					\
> >  	typeof(addr) _addr = (addr);				\
> > -	if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> > -		mmap_end = STACK_TOP_MAX;			\
> 
> Setting mmap_end in the no-hint case seems to have been lost?
> 
> -s

Thanks for catching that, will fix in v2.

- Charlie

> 
> > -	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)))		\
> >  	else							\
> > -		mmap_end = VA_USER_SV39;			\
> > +		mmap_end = (_addr + len);			\
> >  	mmap_end;						\
> >  })
> > 
> > @@ -39,14 +36,12 @@
> >  	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;		\
> >  	mmap_base;						\
> >  })
> > 
> >
> > -- 
> > 2.43.0
> >
> >
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