It is needed to port language runtime with coroutine support. Guard pages are required to handle stack overflow (runtime system can handle stack overflows and resume execution, stack memory is allocated with page probing to prevent bypassing guard page).
Kernel use create_area_etc (usage when creating thread stack), that is not available from userland.
Also it will be nice to have ability to change thread stack area in kernel.
You can just do it “manually”, i.e. by mmap’ing an area before or after the stack area with PROT_NONE, which should work on all POSIX OSes, right?
You can use mprotect() with PROT_NONE to protect a memory page (including one that’s in your stack area) and you can use pthread_attr_setstack to set a manually allocated stack for a specific thread (only when creating it, not after it has started running).
I think it should be possible with these two and mmap to:
So, indeed this should be pretty close to what’s available on any POSIX OS.
This can’t be used for switching coroutines. Windows has nice API for creating and switching thread stacks: Fibers.
What API will be used for reserving address space?
You can use mmap with appropriate flag:
mmap(0, bytes, PROT_NONE, MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);
This creates an address space reservation that you can latter fill in by calling mmap again with PROT_READ | PROT_WRITE and removing the MAP_NORESERVE flag
Example usage in WebKit:
Also what B_STACK_AREA protection means? Is it needed for custom stacks? I tried to set it, it was displayed in SystemManager, but it seems to have no effect.
In src/system/kernel/vm/vm.cpp it seems to be used only for ensuring a minimum size for the area of two pages (the minimum required to have an usable page and a guard page).
It seems to also allow overcommitting.
It is probably best for languages to just implement their own coroutine-switching functions, or use a library like libco for it (one of the backends uses POSIX setjmp/longjmp and thus should be architecture-independent.)
It is currently not possible to change thread stack range (see thread_info stack_base/stack_end) and stack area for running thread. Program may want to exit and delete coroutine that it using default thread stack. Windows fiber API support registering stack virtual memory and stack range in TEB so it behave exactly same as default thread stack and stacks can be deleted including default one.
I tried this approach, separate areas are created for guard pages. This is probably bad because a lot of coroutines can be created.
Test code:
#include <fcntl.h>
#include <sys/mman.h>
#include <stdio.h>
#include <signal.h>
#include <setjmp.h>
#include <OS.h>
struct TryFrame {
sigjmp_buf context;
void *trapAdr;
};
static void SignalHandler(int signal, siginfo_t *signalInfo, ucontext_t *ctx, TryFrame *frame)
{
printf("Signal\n");
frame->trapAdr = signalInfo->si_addr;
siglongjmp(frame->context, 1);
}
int main()
{
size_t size = 16*1024*1024;
void *mem = mmap(0, size, PROT_NONE, MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);
printf("mem: %p\n", mem);
void *allocMem = mmap((void*)((int8*)mem + B_PAGE_SIZE), size - 2*B_PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0);
printf("allocMem: %p\n", allocMem);
TryFrame frame = {0};
struct sigaction action = {0};
action.sa_handler = (__sighandler_t)SignalHandler;
action.sa_flags = SA_SIGINFO;
action.sa_userdata = &frame;
sigaction(SIGSEGV, &action, NULL);
if (sigsetjmp(frame.context, true) == 0) {
for (int8 *ptr = (int8*)mem + size/2; ; ptr--) {
*ptr = 0xcc;
}
} else {
printf("low: 0x%" B_PRIxADDR "\n", (int8*)frame.trapAdr - (int8*)mem);
}
if (sigsetjmp(frame.context, true) == 0) {
for (int8 *ptr = (int8*)mem + size/2; ; ptr++) {
*ptr = 0xcc;
}
} else {
printf("high: 0x%" B_PRIxADDR "\n", ((int8*)mem + size) - (int8*)frame.trapAdr);
}
printf("[WAIT]"); fgetc(stdin);
return 0;
}
list_area:
./MMapTest2 (team 56487)
ID name address size alloc. #-cow #-in #-out
------------------------------------------------------------------------------------
6051848 rld heap 0x46e32aa000 10000 d000 0 0 0
6051849 _rld_debug_ 0x70f71b5000 1000 1000 0 0 0
6051858 MMapTest2 mmap area 0x8046306000 1000 0 0 0 0
6051859 MMapTest2 mmap area 0x8046307000 ffe000 ffe000 0 0 0
6051860 MMapTest2 mmap area 0x8047305000 1000 0 0 0 0
6051850 MMapTest2_seg0ro 0xb156819000 1000 0 0 0 0
6051851 MMapTest2_seg1rw 0xb156a19000 1000 1000 0 0 0
6051846 runtime_loader_seg0ro 0xd440571000 21000 0 0 0 0
6051847 runtime_loader_seg1rw 0xd440791000 2000 2000 0 0 0
6051855 libgcc_s.so.1_seg0ro 0x13753b97000 16000 0 0 0 0
6051856 libgcc_s.so.1_seg1rw 0x13753dac000 1000 1000 0 0 0
6051852 libroot.so_seg0ro 0x141fdabc000 10d000 0 0 0 0
6051853 libroot.so_seg1rw 0x141fddc9000 e000 e000 0 0 0
6051854 libroot.so_seg2rw 0x141fddd7000 44000 6000 0 0 0
6051857 heap 0x10c4ebaa7000 50000 1b000 0 0 0
6051844 MMapTest2_56487_stack 0x7fd732900000 1006000 6000 0 0 0
6051843 user area 0x7ff38a47b000 4000 4000 0 0 0
6051845 commpage 0x7ffff5f4d000 8000 0 0 0 0