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

From: Palmer Dabbelt
Date: Fri Mar 22 2024 - 10:13:48 EST

On Thu, 01 Feb 2024 18:28:06 PST (-0800), Charlie Jenkins wrote:
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@xxxxxxxxxxxx>
> > ---
> >  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.

Sorry I forgot to reply to this, I had a buffer sitting around somewhere but I must have lost it.

I think Charlie's approach is the right way to go. Putting my userspace hat on, I'd much rather have my allocations fail rather than silently ignore the hint when there's memory pressure.

If there's some real use case that needs these low hints to be silently ignored under VA pressure then we can try and figure something out that makes those applications work.


- Charlie