On Mon, Aug 19, 2024 at 01:55:57PM +0800, Levi Zim wrote:
On 2024-03-22 22:06, Palmer Dabbelt wrote:
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@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.
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.
I could confirm that this patch has broken chromium's partition allocator on riscv64. The minimal reproduction I use is chromium-mmap.c:
#include <stdio.h> #include <sys/mman.h>
int main() { void* expected = (void*)0x400000000; void* addr = mmap(expected, 17179869184, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); if (addr != expected) {
It is not valid to assume that the address returned by mmap will be the hint address. If the hint address is not available, mmap will return a different address.
printf("Not expected address: %p != %p\n", addr, expected); } expected = (void*)0x3fffff000; addr = mmap(expected, 17179873280, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); if (addr != expected) { printf("Not expected address: %p != %p\n", addr, expected); } return 0; }
The second mmap fails with ENOMEM. Manually reverting this commit fixes the issue for me. So I think it's clearly a regression and breaks userspace.
The issue here is that overlapping memory is being requested. This second mmap will never be able to provide an address at 0x3fffff000 with a size of 0x400001000 since mmap just provided an address at 0x400000000 with a size of 0x400000000.
Before this patch, this request causes mmap to return a completely arbitrary value. There is no reason to use a hint address in this manner because the hint can never be respected. Since an arbitrary address is desired, a hint of zero should be used.
This patch causes the behavior to be more deterministic. Instead of providing an arbitrary address, it causes the address to be less than or equal to the hint address. This allows for applications to make assumptions about the returned address.
This code is unfortunately relying on the previously mostly undefined behavior of the hint address in mmap. The goal of this patch is to help developers have more consistent mmap behavior, but maybe it is necessary to hide this behavior behind an mmap flag.
- Charlie
See also https://github.com/riscv-forks/electron/issues/4
- Charlie
Sincerely, Levi