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@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) .
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; \
> > })
> >
> >
>
