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.