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

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Jan 31, 2024 at 11:59:43PM +0800, Yangyu Chen wrote:
> On Wed, 2024-01-31 at 22:41 +0800, Yangyu Chen wrote:
> > 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.
> > 
> 
> This is because hint address < rnd_gap. I have tried to let mmap_base =
> min((_addr + len), (base) + TASK_SIZE - DEFAULT_MAP_WINDOW). However it
> does not work for bottom-up while ulimit -s is unlimited. You said this
> behavior is expected from patch v2 review. However it brings a new
> regression even on sv39 systems.
> 
> I still don't know the reason why use addr+len as the upper-bound. I
> think solution like x86/arm64/powerpc provide two address space switch
> based on whether hint address above the default map window is enough.
> 

Yep this is expected. It is up to the maintainers to decide.

- Charlie