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

[Yangyu Chen]

On Mon, 2024-01-29 at 18:50 -0800, Charlie Jenkins wrote:
> On Tue, Jan 30, 2024 at 10:34:03AM +0800, Yangyu Chen wrote:
> > 
> > > On Jan 30, 2024, at 08:37, Charlie Jenkins <charlie@xxxxxxxxxxxx>
> > > 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; \
> > > - 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))) \
> > 
> > How about replacing BIT(VA_BITS-1) to DEFAULT_MAP_WINDOW to make
> > the code
> > more general.
> > 
> > > 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))) \
> > 
> > Same here.
> > 
> > > 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; \
> > > })
> > > 
> > > 
> > 
> > What about not setting the upper bound as x86/arm/powerpc as [1]
> > did?
> > In this case, user space can directly pass a constant hint address
> > >
> > BIT(47) to get a mapping in sv57. If you want this, this code also
> > allows
> > user-space to pass any address larger than TASK_SIZE. You should
> > also
> > limit the mmap_base to (base) + TASK_SIZE - DEFAULT_MAP_WINDOW.
> 
> No. This suggestion causes a random address to be used if the hint
> address is not available. That is why I didn't simply go with your
> patch.


I think return random address is expected and other ISAs like
x86/arm/powerpc will also return random address if hint is NULL.

Also add CC linux-mm to get more opinions from people who familiar with
mm.

> 
> This patch both gives your application the benefit of being able to
> use
> a hint address in the hopes that the address is available, as well as
> continuing to support the guarantee that an address using more bits
> than
> the hint address is not returned.
> 
> - Charlie
> 
> > 
> > I’m also aware of the rnd_gap if it is not 0, then we will not get
> > address mapped to VA_USER_SV39 - rnd_gap.
> > 
> > [1].
> > https://lore.kernel.org/linux-riscv/tencent_2683632BEE438C6D4854E30BDF9CA0843606@xxxxxx/
> > 
> > > -- 
> > > 2.43.0
> > > 
> >