MIT6.828_OS/lab/lib/fork.c

// implement fork from user space

#include <inc/string.h>
#include <inc/lib.h>

// PTE_COW marks copy-on-write page table entries.
// It is one of the bits explicitly allocated to user processes (PTE_AVAIL).
#define PTE_COW		0x800

//
// Custom page fault handler - if faulting page is copy-on-write,
// map in our own private writable copy.
//
static void
pgfault(struct UTrapframe *utf)
{
	void *addr = (void *) utf->utf_fault_va;
	uint32_t err = utf->utf_err;
	int r;

	// Check that the faulting access was (1) a write, and (2) to a
	// copy-on-write page.  If not, panic.
	// Hint:
	//   Use the read-only page table mappings at uvpt
	//   (see <inc/memlayout.h>).

	// LAB 4: Your code here.
	if (! ( (err & FEC_WR) && (uvpd[PDX(addr)] & PTE_P) && (uvpt[PGNUM(addr)] & PTE_P) && (uvpt[PGNUM(addr)] & PTE_COW)))
		panic("Neither the fault is a write nor COW page. \n");
	// Allocate a new page, map it at a temporary location (PFTEMP),
	// copy the data from the old page to the new page, then move the new
	// page to the old page's address.
	// Hint:
	//   You should make three system calls.

	// LAB 4: Your code here.
	envid_t envid = sys_getenvid();
	// cprintf("pgfault: envid: %d\n", ENVX(envid));
	// 临时页暂存
	if ((r = sys_page_alloc(envid, (void *)PFTEMP, PTE_P| PTE_W|PTE_U)) < 0)
		panic("pgfault: page allocation fault:%e\n", r);
	addr = ROUNDDOWN(addr, PGSIZE);
	memcpy((void *) PFTEMP, (const void *) addr, PGSIZE);
	if ((r = sys_page_map(envid, (void *) PFTEMP, envid, addr , PTE_P|PTE_W|PTE_U)) < 0 )
		panic("pgfault: page map failed %e\n", r);
	
	if ((r = sys_page_unmap(envid, (void *) PFTEMP)) < 0)
		panic("pgfault: page unmap failed %e\n", r);

	
		
	// panic("pgfault not implemented");
}

//
// Map our virtual page pn (address pn*PGSIZE) into the target envid
// at the same virtual address.  If the page is writable or copy-on-write,
// the new mapping must be created copy-on-write, and then our mapping must be
// marked copy-on-write as well.  (Exercise: Why do we need to mark ours
// copy-on-write again if it was already copy-on-write at the beginning of
// this function?)
//
// Returns: 0 on success, < 0 on error.
// It is also OK to panic on error.
//

static int
duppage(envid_t envid, unsigned pn)
{

	// LAB 4: Your code here.
	pte_t *pte;
	int ret;
	// 用户空间的地址较低
	uint32_t va = pn * PGSIZE;

	if (uvpt[pn] & PTE_SHARE) {
		if((ret = sys_page_map(thisenv->env_id, (void *) va, envid, (void * )va, uvpt[pn] & PTE_SYSCALL)) <0 ) 
			return ret;
	}
	else if ( (uvpt[pn] & PTE_W) || (uvpt[pn] & PTE_COW)) {
		
		// 子进程标记
		if ((ret = sys_page_map(thisenv->env_id, (void *) va, envid, (void *) va, PTE_P|PTE_U|PTE_COW)) < 0)
			return ret;
		// 父进程标记
		if ((ret = sys_page_map(thisenv->env_id, (void *)va, thisenv->env_id, (void *)va, PTE_P|PTE_U|PTE_COW)) < 0)
			return ret;
	}
	else {
		// 简单映射
		if((ret = sys_page_map(thisenv->env_id, (void *) va, envid, (void * )va, PTE_P|PTE_U)) <0 ) 
			return ret;
	}

	return 0;
	// panic("duppage not implemented");
}


//
// User-level fork with copy-on-write.
// Set up our page fault handler appropriately.
// Create a child.
// Copy our address space and page fault handler setup to the child.
// Then mark the child as runnable and return.
//
// Returns: child's envid to the parent, 0 to the child, < 0 on error.
// It is also OK to panic on error.
//
// Hint:
//   Use uvpd, uvpt, and duppage.
//   Remember to fix "thisenv" in the child process.
//   Neither user exception stack should ever be marked copy-on-write,
//   so you must allocate a new page for the child's user exception stack.
//
envid_t
fork(void)
{
	// LAB 4: Your code here.
	envid_t envid;
	int r;
	size_t i, j, pn;
	// Set up our page fault handler
    set_pgfault_handler(pgfault);
	
	envid = sys_exofork();
	
	if (envid < 0) {
		panic("sys_exofork failed: %e", envid);
	}
	
	if (envid == 0) {
		// child
		thisenv = &envs[ENVX(sys_getenvid())];
		return 0;
	}
	// here is parent !
	// Copy our address space and page fault handler setup to the child.
	
	for (pn = PGNUM(UTEXT); pn < PGNUM(USTACKTOP); pn++) {
		if ( (uvpd[pn >> 10] & PTE_P) && (uvpt[pn] & PTE_P)) {
			// 页表
			if ( (r = duppage(envid, pn)) < 0)
				return r;
			
		}
	}
	// alloc a page and map child exception stack
    if ((r = sys_page_alloc(envid, (void *)(UXSTACKTOP-PGSIZE), PTE_U | PTE_P | PTE_W)) < 0)
        return r;
    extern void _pgfault_upcall(void);
    if ((r = sys_env_set_pgfault_upcall(envid, _pgfault_upcall)) < 0)
	return r;

    // Start the child environment running
    if ((r = sys_env_set_status(envid, ENV_RUNNABLE)) < 0)
	panic("sys_env_set_status: %e", r);
	
    return envid;
	// panic("fork not implemented");
}

// Challenge!
int
sfork(void)
{
	panic("sfork not implemented");
	return -E_INVAL;
}