X-Git-Url: http://plrg.eecs.uci.edu/git/?p=c11tester.git;a=blobdiff_plain;f=snapshot.cc;h=d73613832e867eb069b5f5288b72f95bc04c96ff;hp=99b9d7958753d49b77513e279ff40bd4196ad178;hb=1b2081fd7a99dd894f75b8606073d3eae57a0013;hpb=7d2149ffbbc1a3514838d721a71bb0d4c81054b3

diff --git a/snapshot.cc b/snapshot.cc
index 99b9d795..d7361383 100644
--- a/snapshot.cc
+++ b/snapshot.cc
@@ -3,325 +3,479 @@
 #include <unistd.h>
 #include <signal.h>
 #include <stdlib.h>
-#include <map>
-#include <cstring>
-#include <cstdio>
-#include "snapshot.h"
-#include "snapshotimp.h"
-#include "mymemory.h"
-#include <fcntl.h>
-#include <assert.h>
-#include <pthread.h>
-#include <semaphore.h>
+#include <string.h>
 #include <errno.h>
 #include <sys/wait.h>
-#include <ucontext.h>
-
-#define FAILURE(mesg) { printf("failed in the API: %s with errno relative message: %s\n", mesg, strerror( errno ) ); exit(EXIT_FAILURE); }
-
-#ifdef CONFIG_SSDEBUG
-#define SSDEBUG		printf
-#else
-#define SSDEBUG(...)	do { } while (0)
-#endif
 
-/* extern declaration definition */
-#if USE_MPROTECT_SNAPSHOT
-struct SnapShot * snapshotrecord = NULL;
-#else
-struct Snapshot * sTheRecord = NULL;
-#endif
-
-#if !USE_MPROTECT_SNAPSHOT
-static ucontext_t savedSnapshotContext;
-static ucontext_t savedUserSnapshotContext;
-static snapshot_id snapshotid = 0;
-#endif
+#include "hashtable.h"
+#include "snapshot.h"
+#include "mymemory.h"
+#include "common.h"
+#include "context.h"
+#include "model.h"
 
 /** PageAlignedAdressUpdate return a page aligned address for the
  * address being added as a side effect the numBytes are also changed.
  */
-static void * PageAlignAddressUpward(void * addr) {
-	return (void *)((((uintptr_t)addr)+PAGESIZE-1)&~(PAGESIZE-1));
+static void * PageAlignAddressUpward(void *addr)
+{
+	return (void *)((((uintptr_t)addr) + PAGESIZE - 1) & ~(PAGESIZE - 1));
 }
 
 #if USE_MPROTECT_SNAPSHOT
 
+/* Each SnapShotRecord lists the firstbackingpage that must be written to
+ * revert to that snapshot */
+struct SnapShotRecord {
+	unsigned int firstBackingPage;
+};
+
+/** @brief Backing store page */
+typedef unsigned char snapshot_page_t[PAGESIZE];
+
+/* List the base address of the corresponding page in the backing store so we
+ * know where to copy it to */
+struct BackingPageRecord {
+	void *basePtrOfPage;
+};
+
+/* Struct for each memory region */
+struct MemoryRegion {
+	void *basePtr;	// base of memory region
+	int sizeInPages;	// size of memory region in pages
+};
+
 /** ReturnPageAlignedAddress returns a page aligned address for the
  * address being added as a side effect the numBytes are also changed.
  */
-static void * ReturnPageAlignedAddress(void * addr) {
-	return (void *)(((uintptr_t)addr)&~(PAGESIZE-1));
+static void * ReturnPageAlignedAddress(void *addr)
+{
+	return (void *)(((uintptr_t)addr) & ~(PAGESIZE - 1));
 }
 
-/** The initSnapShotRecord method initialized the snapshotting data
- *  structures for the mprotect based snapshot. 
- */
-void initSnapShotRecord(unsigned int numbackingpages, unsigned int numsnapshots, unsigned int nummemoryregions) {
-	snapshotrecord=( struct SnapShot * )MYMALLOC(sizeof(struct SnapShot));
-	snapshotrecord->regionsToSnapShot=( struct MemoryRegion * )MYMALLOC(sizeof(struct MemoryRegion)*nummemoryregions);
-	snapshotrecord->backingStoreBasePtr= ( struct SnapShotPage * )MYMALLOC( sizeof( struct SnapShotPage ) * (numbackingpages + 1) );
+/* Primary struct for snapshotting system */
+struct mprot_snapshotter {
+	mprot_snapshotter(unsigned int numbackingpages, unsigned int numsnapshots, unsigned int nummemoryregions);
+	~mprot_snapshotter();
+
+	struct MemoryRegion *regionsToSnapShot;	//This pointer references an array of memory regions to snapshot
+	snapshot_page_t *backingStore;	//This pointer references an array of snapshotpage's that form the backing store
+	void *backingStoreBasePtr;	//This pointer references an array of snapshotpage's that form the backing store
+	struct BackingPageRecord *backingRecords;	//This pointer references an array of backingpagerecord's (same number of elements as backingstore
+	struct SnapShotRecord *snapShots;	//This pointer references the snapshot array
+
+	unsigned int lastSnapShot;	//Stores the next snapshot record we should use
+	unsigned int lastBackingPage;	//Stores the next backingpage we should use
+	unsigned int lastRegion;	//Stores the next memory region to be used
+
+	unsigned int maxRegions;	//Stores the max number of memory regions we support
+	unsigned int maxBackingPages;	//Stores the total number of backing pages
+	unsigned int maxSnapShots;	//Stores the total number of snapshots we allow
+
+	MEMALLOC
+};
+
+static struct mprot_snapshotter *mprot_snap = NULL;
+
+mprot_snapshotter::mprot_snapshotter(unsigned int backing_pages, unsigned int snapshots, unsigned int regions) :
+	lastSnapShot(0),
+	lastBackingPage(0),
+	lastRegion(0),
+	maxRegions(regions),
+	maxBackingPages(backing_pages),
+	maxSnapShots(snapshots)
+{
+	regionsToSnapShot = (struct MemoryRegion *)model_malloc(sizeof(struct MemoryRegion) * regions);
+	backingStoreBasePtr = (void *)model_malloc(sizeof(snapshot_page_t) * (backing_pages + 1));
 	//Page align the backingstorepages
-	snapshotrecord->backingStore=( struct SnapShotPage * )PageAlignAddressUpward(snapshotrecord->backingStoreBasePtr);
-	snapshotrecord->backingRecords=( struct BackingPageRecord * )MYMALLOC(sizeof(struct BackingPageRecord)*numbackingpages);
-	snapshotrecord->snapShots= ( struct SnapShotRecord * )MYMALLOC(sizeof(struct SnapShotRecord)*numsnapshots);
-	snapshotrecord->lastSnapShot=0;
-	snapshotrecord->lastBackingPage=0;
-	snapshotrecord->lastRegion=0;
-	snapshotrecord->maxRegions=nummemoryregions;
-	snapshotrecord->maxBackingPages=numbackingpages;
-	snapshotrecord->maxSnapShots=numsnapshots;
+	backingStore = (snapshot_page_t *)PageAlignAddressUpward(backingStoreBasePtr);
+	backingRecords = (struct BackingPageRecord *)model_malloc(sizeof(struct BackingPageRecord) * backing_pages);
+	snapShots = (struct SnapShotRecord *)model_malloc(sizeof(struct SnapShotRecord) * snapshots);
 }
-#endif //nothing to initialize for the fork based snapshotting.
 
-/** HandlePF is the page fault handler for mprotect based snapshotting
+mprot_snapshotter::~mprot_snapshotter()
+{
+	model_free(regionsToSnapShot);
+	model_free(backingStoreBasePtr);
+	model_free(backingRecords);
+	model_free(snapShots);
+}
+
+/** mprot_handle_pf is the page fault handler for mprotect based snapshotting
  * algorithm.
  */
-
-void HandlePF( int sig, siginfo_t *si, void * unused){
-#if USE_MPROTECT_SNAPSHOT
-	if( si->si_code == SEGV_MAPERR ){
-		printf("Real Fault at %p\n", si->si_addr);
-		exit( EXIT_FAILURE );
+static void mprot_handle_pf(int sig, siginfo_t *si, void *unused)
+{
+	if (si->si_code == SEGV_MAPERR) {
+		model_print("Segmentation fault at %p\n", si->si_addr);
+		model_print("For debugging, place breakpoint at: %s:%d\n",
+								__FILE__, __LINE__);
+		// print_trace(); // Trace printing may cause dynamic memory allocation
+		exit(EXIT_FAILURE);
 	}
 	void* addr = ReturnPageAlignedAddress(si->si_addr);
 
-	unsigned int backingpage=snapshotrecord->lastBackingPage++; //Could run out of pages...
-	if (backingpage==snapshotrecord->maxBackingPages) {
-		printf("Out of backing pages at %p\n", si->si_addr);
-		exit( EXIT_FAILURE );
+	unsigned int backingpage = mprot_snap->lastBackingPage++;	//Could run out of pages...
+	if (backingpage == mprot_snap->maxBackingPages) {
+		model_print("Out of backing pages at %p\n", si->si_addr);
+		exit(EXIT_FAILURE);
 	}
 
 	//copy page
-	memcpy(&(snapshotrecord->backingStore[backingpage]), addr, sizeof(struct SnapShotPage));
+	memcpy(&(mprot_snap->backingStore[backingpage]), addr, sizeof(snapshot_page_t));
 	//remember where to copy page back to
-	snapshotrecord->backingRecords[backingpage].basePtrOfPage=addr;
+	mprot_snap->backingRecords[backingpage].basePtrOfPage = addr;
 	//set protection to read/write
-	if (mprotect( addr, sizeof(struct SnapShotPage), PROT_READ | PROT_WRITE )) {
+	if (mprotect(addr, sizeof(snapshot_page_t), PROT_READ | PROT_WRITE)) {
 		perror("mprotect");
 		// Handle error by quitting?
 	}
-#endif //nothing to handle for non snapshotting case.
-}
-
-void createSharedLibrary(){
-#if !USE_MPROTECT_SNAPSHOT
-	//step 1. create shared memory.
-	if( sTheRecord ) return;
-	int fd = shm_open( "/ModelChecker-Snapshotter", O_RDWR | O_CREAT, 0777 ); //universal permissions.
-	if( -1 == fd ) FAILURE("shm_open");
-	if( -1 == ftruncate( fd, ( size_t )SHARED_MEMORY_DEFAULT + ( size_t )STACK_SIZE_DEFAULT ) ) FAILURE( "ftruncate" );
-	char * memMapBase = ( char * ) mmap( 0, ( size_t )SHARED_MEMORY_DEFAULT + ( size_t )STACK_SIZE_DEFAULT, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0 );
-	if( MAP_FAILED == memMapBase ) FAILURE("mmap");
-	sTheRecord = ( struct Snapshot * )memMapBase;
-	sTheRecord->mSharedMemoryBase = memMapBase + sizeof( struct Snapshot );
-	sTheRecord->mStackBase = ( char * )memMapBase + ( size_t )SHARED_MEMORY_DEFAULT;
-	sTheRecord->mStackSize = STACK_SIZE_DEFAULT;
-	sTheRecord->mIDToRollback = -1;
-	sTheRecord->currSnapShotID = 0;
-	sTheRecord->mbFinalize = false;
-#endif
 }
 
-/** The initSnapShotLibrary function initializes the Snapshot library.
- *  @param entryPoint the function that should run the program.
- */
-void initSnapShotLibrary(unsigned int numbackingpages,
-		unsigned int numsnapshots, unsigned int nummemoryregions,
-		unsigned int numheappages, VoidFuncPtr entryPoint) {
-#if USE_MPROTECT_SNAPSHOT
+static void mprot_snapshot_init(unsigned int numbackingpages,
+																unsigned int numsnapshots, unsigned int nummemoryregions,
+																unsigned int numheappages, VoidFuncPtr entryPoint)
+{
 	/* Setup a stack for our signal handler....  */
 	stack_t ss;
-	ss.ss_sp = MYMALLOC(SIGSTACKSIZE);
+	ss.ss_sp = PageAlignAddressUpward(model_malloc(SIGSTACKSIZE + PAGESIZE - 1));
 	ss.ss_size = SIGSTACKSIZE;
 	ss.ss_flags = 0;
 	sigaltstack(&ss, NULL);
 
 	struct sigaction sa;
 	sa.sa_flags = SA_SIGINFO | SA_NODEFER | SA_RESTART | SA_ONSTACK;
-	sigemptyset( &sa.sa_mask );
-	sa.sa_sigaction = HandlePF;
+	sigemptyset(&sa.sa_mask);
+	sa.sa_sigaction = mprot_handle_pf;
 #ifdef MAC
-	if( sigaction( SIGBUS, &sa, NULL ) == -1 ){
-		printf("SIGACTION CANNOT BE INSTALLED\n");
+	if (sigaction(SIGBUS, &sa, NULL) == -1) {
+		perror("sigaction(SIGBUS)");
 		exit(EXIT_FAILURE);
 	}
 #endif
-	if( sigaction( SIGSEGV, &sa, NULL ) == -1 ){
-		printf("SIGACTION CANNOT BE INSTALLED\n");
+	if (sigaction(SIGSEGV, &sa, NULL) == -1) {
+		perror("sigaction(SIGSEGV)");
 		exit(EXIT_FAILURE);
 	}
 
-	initSnapShotRecord(numbackingpages, numsnapshots, nummemoryregions);
+	mprot_snap = new mprot_snapshotter(numbackingpages, numsnapshots, nummemoryregions);
 
-	// EVIL HACK: We need to make sure that calls into the HandlePF method don't cause dynamic links
+	// EVIL HACK: We need to make sure that calls into the mprot_handle_pf method don't cause dynamic links
 	// The problem is that we end up protecting state in the dynamic linker...
 	// Solution is to call our signal handler before we start protecting stuff...
 
 	siginfo_t si;
 	memset(&si, 0, sizeof(si));
-	si.si_addr=ss.ss_sp;
-	HandlePF(SIGSEGV, &si, NULL);
-	snapshotrecord->lastBackingPage--; //remove the fake page we copied
-
-	basemySpace=MYMALLOC((numheappages+1)*PAGESIZE);
-	void * pagealignedbase=PageAlignAddressUpward(basemySpace);
-	mySpace = create_mspace_with_base(pagealignedbase,  numheappages*PAGESIZE, 1 );
-	addMemoryRegionToSnapShot(pagealignedbase, numheappages);
-	entryPoint();
-#else
+	si.si_addr = ss.ss_sp;
+	mprot_handle_pf(SIGSEGV, &si, NULL);
+	mprot_snap->lastBackingPage--;	//remove the fake page we copied
 
-	basemySpace=system_malloc((numheappages+1)*PAGESIZE);
-	void * pagealignedbase=PageAlignAddressUpward(basemySpace);
-	mySpace = create_mspace_with_base(pagealignedbase,  numheappages*PAGESIZE, 1 );
-	createSharedLibrary();
-
-	//step 2 setup the stack context.
-
-	int alreadySwapped = 0;
-	getcontext( &savedSnapshotContext );
-	if( !alreadySwapped ){
-		alreadySwapped = 1;
-		ucontext_t currentContext, swappedContext, newContext;
-		getcontext( &newContext );
-		newContext.uc_stack.ss_sp = sTheRecord->mStackBase;
-		newContext.uc_stack.ss_size = STACK_SIZE_DEFAULT;
-		newContext.uc_link = &currentContext;
-		makecontext( &newContext, entryPoint, 0 );
-		swapcontext( &swappedContext, &newContext );
-	}
+	void *basemySpace = model_malloc((numheappages + 1) * PAGESIZE);
+	void *pagealignedbase = PageAlignAddressUpward(basemySpace);
+	user_snapshot_space = create_mspace_with_base(pagealignedbase, numheappages * PAGESIZE, 1);
+	snapshot_add_memory_region(pagealignedbase, numheappages);
 
-	//add the code to take a snapshot here...
-	//to return to user process, do a second swapcontext...
-	pid_t forkedID = 0;
-	snapshotid = sTheRecord->currSnapShotID;
-	bool swapContext = false;
-	while( !sTheRecord->mbFinalize ){
-		sTheRecord->currSnapShotID=snapshotid+1;
-		forkedID = fork();
-		if( 0 == forkedID ){
-			ucontext_t currentContext;
-#if 0
-			int dbg = 0;
-			while( !dbg );
-#endif
-			if( swapContext )
-				swapcontext( &currentContext, &( sTheRecord->mContextToRollback ) );
-			else{
-				swapcontext( &currentContext, &savedUserSnapshotContext );
-			}
-		} else {
-			int status;
-			int retVal;
-
-			SSDEBUG("The process id of child is %d and the process id of this process is %d and snapshot id is %d",
-			        forkedID, getpid(), snapshotid );
+	void *base_model_snapshot_space = model_malloc((numheappages + 1) * PAGESIZE);
+	pagealignedbase = PageAlignAddressUpward(base_model_snapshot_space);
+	model_snapshot_space = create_mspace_with_base(pagealignedbase, numheappages * PAGESIZE, 1);
+	snapshot_add_memory_region(pagealignedbase, numheappages);
 
-			do {
-				retVal=waitpid( forkedID, &status, 0 );
-			} while( -1 == retVal && errno == EINTR );
-
-			if( sTheRecord->mIDToRollback != snapshotid )
-				exit(EXIT_SUCCESS);
-			else{
-				swapContext = true;
-			}
-		}
-	}
-
-#endif
+	entryPoint();
 }
 
-/** The addMemoryRegionToSnapShot function assumes that addr is page aligned. 
- */
-void addMemoryRegionToSnapShot( void * addr, unsigned int numPages) {
-#if USE_MPROTECT_SNAPSHOT
-	unsigned int memoryregion=snapshotrecord->lastRegion++;
-	if (memoryregion==snapshotrecord->maxRegions) {
-		printf("Exceeded supported number of memory regions!\n");
+static void mprot_add_to_snapshot(void *addr, unsigned int numPages)
+{
+	unsigned int memoryregion = mprot_snap->lastRegion++;
+	if (memoryregion == mprot_snap->maxRegions) {
+		model_print("Exceeded supported number of memory regions!\n");
 		exit(EXIT_FAILURE);
 	}
 
-	snapshotrecord->regionsToSnapShot[ memoryregion ].basePtr=addr;
-	snapshotrecord->regionsToSnapShot[ memoryregion ].sizeInPages=numPages;
-#endif //NOT REQUIRED IN THE CASE OF FORK BASED SNAPSHOTS.
+	DEBUG("snapshot region %p-%p (%u page%s)\n",
+				addr, (char *)addr + numPages * PAGESIZE, numPages,
+				numPages > 1 ? "s" : "");
+	mprot_snap->regionsToSnapShot[memoryregion].basePtr = addr;
+	mprot_snap->regionsToSnapShot[memoryregion].sizeInPages = numPages;
 }
 
-/** The takeSnapshot function takes a snapshot.
- * @return The snapshot identifier.
- */
-
-snapshot_id takeSnapshot( ){
-#if USE_MPROTECT_SNAPSHOT
-	for(unsigned int region=0; region<snapshotrecord->lastRegion;region++) {
-		if( mprotect(snapshotrecord->regionsToSnapShot[region].basePtr, snapshotrecord->regionsToSnapShot[region].sizeInPages*sizeof(struct SnapShotPage), PROT_READ ) == -1 ){
+static snapshot_id mprot_take_snapshot()
+{
+	for (unsigned int region = 0;region < mprot_snap->lastRegion;region++) {
+		if (mprotect(mprot_snap->regionsToSnapShot[region].basePtr, mprot_snap->regionsToSnapShot[region].sizeInPages * sizeof(snapshot_page_t), PROT_READ) == -1) {
 			perror("mprotect");
-			printf("Failed to mprotect inside of takeSnapShot\n");
+			model_print("Failed to mprotect inside of takeSnapShot\n");
 			exit(EXIT_FAILURE);
 		}
 	}
-	unsigned int snapshot=snapshotrecord->lastSnapShot++;
-	if (snapshot==snapshotrecord->maxSnapShots) {
-		printf("Out of snapshots\n");
+	unsigned int snapshot = mprot_snap->lastSnapShot++;
+	if (snapshot == mprot_snap->maxSnapShots) {
+		model_print("Out of snapshots\n");
 		exit(EXIT_FAILURE);
 	}
-	snapshotrecord->snapShots[snapshot].firstBackingPage=snapshotrecord->lastBackingPage;
+	mprot_snap->snapShots[snapshot].firstBackingPage = mprot_snap->lastBackingPage;
 
 	return snapshot;
-#else
-	swapcontext( &savedUserSnapshotContext, &savedSnapshotContext );
-	return snapshotid;
-#endif
 }
 
-/** The rollBack function rollback to the given snapshot identifier.
- *  @param theID is the snapshot identifier to rollback to.
- */
+static void mprot_roll_back(snapshot_id theID)
+{
+#if USE_MPROTECT_SNAPSHOT == 2
+	if (mprot_snap->lastSnapShot == (theID + 1)) {
+		for (unsigned int page = mprot_snap->snapShots[theID].firstBackingPage;page < mprot_snap->lastBackingPage;page++) {
+			memcpy(mprot_snap->backingRecords[page].basePtrOfPage, &mprot_snap->backingStore[page], sizeof(snapshot_page_t));
+		}
+		return;
+	}
+#endif
 
-void rollBack( snapshot_id theID ){
-#if USE_MPROTECT_SNAPSHOT
-	std::map< void *, bool, std::less< void * >, MyAlloc< std::pair< const void *, bool > > > duplicateMap;
-	for(unsigned int region=0; region<snapshotrecord->lastRegion;region++) {
-		if( mprotect(snapshotrecord->regionsToSnapShot[region].basePtr, snapshotrecord->regionsToSnapShot[region].sizeInPages*sizeof(struct SnapShotPage), PROT_READ | PROT_WRITE ) == -1 ){
+	HashTable< void *, bool, uintptr_t, 4, model_malloc, model_calloc, model_free> duplicateMap;
+	for (unsigned int region = 0;region < mprot_snap->lastRegion;region++) {
+		if (mprotect(mprot_snap->regionsToSnapShot[region].basePtr, mprot_snap->regionsToSnapShot[region].sizeInPages * sizeof(snapshot_page_t), PROT_READ | PROT_WRITE) == -1) {
 			perror("mprotect");
-			printf("Failed to mprotect inside of takeSnapShot\n");
+			model_print("Failed to mprotect inside of takeSnapShot\n");
 			exit(EXIT_FAILURE);
 		}
 	}
-	for(unsigned int page=snapshotrecord->snapShots[theID].firstBackingPage; page<snapshotrecord->lastBackingPage; page++) {
-		bool oldVal = false;
-		if( duplicateMap.find( snapshotrecord->backingRecords[page].basePtrOfPage ) != duplicateMap.end() ){
-			oldVal = true;
+	for (unsigned int page = mprot_snap->snapShots[theID].firstBackingPage;page < mprot_snap->lastBackingPage;page++) {
+		if (!duplicateMap.contains(mprot_snap->backingRecords[page].basePtrOfPage)) {
+			duplicateMap.put(mprot_snap->backingRecords[page].basePtrOfPage, true);
+			memcpy(mprot_snap->backingRecords[page].basePtrOfPage, &mprot_snap->backingStore[page], sizeof(snapshot_page_t));
 		}
-		else{
-			duplicateMap[ snapshotrecord->backingRecords[page].basePtrOfPage ] = true;
-		}
-		if(  !oldVal ){
-			memcpy(snapshotrecord->backingRecords[page].basePtrOfPage, &snapshotrecord->backingStore[page], sizeof(struct SnapShotPage));
+	}
+	mprot_snap->lastSnapShot = theID;
+	mprot_snap->lastBackingPage = mprot_snap->snapShots[theID].firstBackingPage;
+	mprot_take_snapshot();	//Make sure current snapshot is still good...All later ones are cleared
+}
+
+#else	/* !USE_MPROTECT_SNAPSHOT */
+
+#define SHARED_MEMORY_DEFAULT  (200 * ((size_t)1 << 20))	// 100mb for the shared memory
+#define STACK_SIZE_DEFAULT      (((size_t)1 << 20) * 20)	// 20 mb out of the above 100 mb for my stack
+
+struct fork_snapshotter {
+	/** @brief Pointer to the shared (non-snapshot) memory heap base
+	 * (NOTE: this has size SHARED_MEMORY_DEFAULT - sizeof(*fork_snap)) */
+	void *mSharedMemoryBase;
+
+	/** @brief Pointer to the shared (non-snapshot) stack region */
+	void *mStackBase;
+
+	/** @brief Size of the shared stack */
+	size_t mStackSize;
+
+	/**
+	 * @brief Stores the ID that we are attempting to roll back to
+	 *
+	 * Used in inter-process communication so that each process can
+	 * determine whether or not to take over execution (w/ matching ID) or
+	 * exit (we're rolling back even further). Dubiously marked 'volatile'
+	 * to prevent compiler optimizations from messing with the
+	 * inter-process behavior.
+	 */
+	volatile snapshot_id mIDToRollback;
+
+	/**
+	 * @brief The context for the shared (non-snapshot) stack
+	 *
+	 * This context is passed between the various processes which represent
+	 * various snapshot states. It should be used primarily for the
+	 * "client-side" code, not the main snapshot loop.
+	 */
+	ucontext_t shared_ctxt;
+
+	/** @brief Inter-process tracking of the next snapshot ID */
+	snapshot_id currSnapShotID;
+};
+
+static struct fork_snapshotter *fork_snap = NULL;
+
+/** @statics
+ *   These variables are necessary because the stack is shared region and
+ *   there exists a race between all processes executing the same function.
+ *   To avoid the problem above, we require variables allocated in 'safe' regions.
+ *   The bug was actually observed with the forkID, these variables below are
+ *   used to indicate the various contexts to which to switch to.
+ *
+ *   @private_ctxt: the context which is internal to the current process. Used
+ *   for running the internal snapshot/rollback loop.
+ *   @exit_ctxt: a special context used just for exiting from a process (so we
+ *   can use swapcontext() instead of setcontext() + hacks)
+ *   @snapshotid: it is a running counter for the various forked processes
+ *   snapshotid. it is incremented and set in a persistently shared record
+ */
+static ucontext_t private_ctxt;
+static ucontext_t exit_ctxt;
+static snapshot_id snapshotid = 0;
+
+/**
+ * @brief Create a new context, with a given stack and entry function
+ * @param ctxt The context structure to fill
+ * @param stack The stack to run the new context in
+ * @param stacksize The size of the stack
+ * @param func The entry point function for the context
+ */
+static void create_context(ucontext_t *ctxt, void *stack, size_t stacksize,
+													 void (*func)(void))
+{
+	getcontext(ctxt);
+	ctxt->uc_stack.ss_sp = stack;
+	ctxt->uc_stack.ss_size = stacksize;
+	makecontext(ctxt, func, 0);
+}
+
+/** @brief An empty function, used for an "empty" context which just exits a
+ *  process */
+static void fork_exit()
+{
+	/* Intentionally empty */
+}
+
+static void createSharedMemory()
+{
+	//step 1. create shared memory.
+	void *memMapBase = mmap(0, SHARED_MEMORY_DEFAULT + STACK_SIZE_DEFAULT, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
+	if (memMapBase == MAP_FAILED) {
+		perror("mmap");
+		exit(EXIT_FAILURE);
+	}
+
+	//Setup snapshot record at top of free region
+	fork_snap = (struct fork_snapshotter *)memMapBase;
+	fork_snap->mSharedMemoryBase = (void *)((uintptr_t)memMapBase + sizeof(*fork_snap));
+	fork_snap->mStackBase = (void *)((uintptr_t)memMapBase + SHARED_MEMORY_DEFAULT);
+	fork_snap->mStackSize = STACK_SIZE_DEFAULT;
+	fork_snap->mIDToRollback = -1;
+	fork_snap->currSnapShotID = 0;
+}
+
+/**
+ * Create a new mspace pointer for the non-snapshotting (i.e., inter-process
+ * shared) memory region. Only for fork-based snapshotting.
+ *
+ * @return The shared memory mspace
+ */
+mspace create_shared_mspace()
+{
+	if (!fork_snap)
+		createSharedMemory();
+	return create_mspace_with_base((void *)(fork_snap->mSharedMemoryBase), SHARED_MEMORY_DEFAULT - sizeof(*fork_snap), 1);
+}
+
+static void fork_snapshot_init(unsigned int numbackingpages,
+															 unsigned int numsnapshots, unsigned int nummemoryregions,
+															 unsigned int numheappages, VoidFuncPtr entryPoint)
+{
+	if (!fork_snap)
+		createSharedMemory();
+
+	void *base_model_snapshot_space = malloc((numheappages + 1) * PAGESIZE);
+	void *pagealignedbase = PageAlignAddressUpward(base_model_snapshot_space);
+	model_snapshot_space = create_mspace_with_base(pagealignedbase, numheappages * PAGESIZE, 1);
+
+	/* setup an "exiting" context */
+	char stack[128];
+	create_context(&exit_ctxt, stack, sizeof(stack), fork_exit);
+
+	/* setup the shared-stack context */
+	create_context(&fork_snap->shared_ctxt, fork_snap->mStackBase,
+								 STACK_SIZE_DEFAULT, entryPoint);
+	/* switch to a new entryPoint context, on a new stack */
+	model_swapcontext(&private_ctxt, &fork_snap->shared_ctxt);
+
+	/* switch back here when takesnapshot is called */
+	snapshotid = fork_snap->currSnapShotID;
+	if (model->params.nofork) {
+	  setcontext(&fork_snap->shared_ctxt);
+	  exit(EXIT_SUCCESS);
+	}
+	
+	while (true) {
+		pid_t forkedID;
+		fork_snap->currSnapShotID = snapshotid + 1;
+		forkedID = fork();
+
+		if (0 == forkedID) {
+			setcontext(&fork_snap->shared_ctxt);
+		} else {
+			DEBUG("parent PID: %d, child PID: %d, snapshot ID: %d\n",
+						getpid(), forkedID, snapshotid);
+
+			while (waitpid(forkedID, NULL, 0) < 0) {
+				/* waitpid() may be interrupted */
+				if (errno != EINTR) {
+					perror("waitpid");
+					exit(EXIT_FAILURE);
+				}
+			}
+
+			if (fork_snap->mIDToRollback != snapshotid)
+				exit(EXIT_SUCCESS);
 		}
 	}
-	snapshotrecord->lastSnapShot=theID;
-	snapshotrecord->lastBackingPage=snapshotrecord->snapShots[theID].firstBackingPage;
-	takeSnapshot(); //Make sure current snapshot is still good...All later ones are cleared
+}
+
+static snapshot_id fork_take_snapshot()
+{
+	model_swapcontext(&fork_snap->shared_ctxt, &private_ctxt);
+	DEBUG("TAKESNAPSHOT RETURN\n");
+	return snapshotid;
+}
+
+static void fork_roll_back(snapshot_id theID)
+{
+	DEBUG("Rollback\n");
+	fork_snap->mIDToRollback = theID;
+	model_swapcontext(&fork_snap->shared_ctxt, &exit_ctxt);
+	fork_snap->mIDToRollback = -1;
+}
+
+#endif	/* !USE_MPROTECT_SNAPSHOT */
+
+/**
+ * @brief Initializes the snapshot system
+ * @param entryPoint the function that should run the program.
+ */
+void snapshot_system_init(unsigned int numbackingpages,
+													unsigned int numsnapshots, unsigned int nummemoryregions,
+													unsigned int numheappages, VoidFuncPtr entryPoint)
+{
+#if USE_MPROTECT_SNAPSHOT
+	mprot_snapshot_init(numbackingpages, numsnapshots, nummemoryregions, numheappages, entryPoint);
 #else
-	sTheRecord->mIDToRollback = theID;
-	int sTemp = 0;
-	getcontext( &sTheRecord->mContextToRollback );
-	if( !sTemp ){
-		sTemp = 1;
-		SSDEBUG("Invoked rollback");
-		exit(EXIT_SUCCESS);
-	}
+	fork_snapshot_init(numbackingpages, numsnapshots, nummemoryregions, numheappages, entryPoint);
 #endif
 }
 
-/** The finalize method shuts down the snapshotting system.  */
-//Subramanian -- remove this function from the external interface and
-//have us call it internally
+/** Assumes that addr is page aligned. */
+void snapshot_add_memory_region(void *addr, unsigned int numPages)
+{
+#if USE_MPROTECT_SNAPSHOT
+	mprot_add_to_snapshot(addr, numPages);
+#else
+	/* not needed for fork-based snapshotting */
+#endif
+}
 
-void finalize(){
-#if !USE_MPROTECT_SNAPSHOT
-	sTheRecord->mbFinalize = true;
+/** Takes a snapshot of memory.
+ * @return The snapshot identifier.
+ */
+snapshot_id take_snapshot()
+{
+#if USE_MPROTECT_SNAPSHOT
+	return mprot_take_snapshot();
+#else
+	return fork_take_snapshot();
 #endif
 }
 
+/** Rolls the memory state back to the given snapshot identifier.
+ *  @param theID is the snapshot identifier to rollback to.
+ */
+void snapshot_roll_back(snapshot_id theID)
+{
+#if USE_MPROTECT_SNAPSHOT
+	mprot_roll_back(theID);
+#else
+	fork_roll_back(theID);
+#endif
+}