Fix /proc/PID/stack in the new per-process page directory world.

I added an RAII helper called OtherTaskPagingScope. While present,
it switches the kernel over to using another task's page directory.
This is perfect for e.g walking the stack in /proc/PID/stack.

Kernel/MemoryManager.cpp

@@ -346,20 +346,6 @@ bool MemoryManager::unmapSubregion(Task& task, Task::Subregion& subregion)
     return true;
 }
 
-bool MemoryManager::unmapRegionsForTask(Task& task)
-{
-    ASSERT_INTERRUPTS_DISABLED();
-    for (auto& region : task.m_regions) {
-        if (!unmapRegion(task, *region))
-            return false;
-    }
-    for (auto& subregion : task.m_subregions) {
-        if (!unmapSubregion(task, *subregion))
-            return false;
-    }
-    return true;
-}
-
 bool MemoryManager::mapSubregion(Task& task, Task::Subregion& subregion)
 {
     InterruptDisabler disabler;

Kernel/MemoryManager.h

@@ -54,13 +54,10 @@ public:
 
     bool mapSubregion(Task&, Task::Subregion&);
     bool unmapSubregion(Task&, Task::Subregion&);
-    bool mapSubregionsForTask(Task&);
-    bool unmapSubregionsForTask(Task&);
 
     bool mapRegion(Task&, Task::Region&);
     bool unmapRegion(Task&, Task::Region&);
     bool mapRegionsForTask(Task&);
-    bool unmapRegionsForTask(Task&);
 
     void registerZone(Zone&);
     void unregisterZone(Zone&);
@@ -182,3 +179,13 @@ private:
 
     Vector<PhysicalAddress> m_freePages;
 };
+
+struct KernelPagingScope {
+    KernelPagingScope() { MM.enter_kernel_paging_scope(); }
+    ~KernelPagingScope() { MM.enter_task_paging_scope(*current); }
+};
+
+struct OtherTaskPagingScope {
+    OtherTaskPagingScope(Task& task) { MM.enter_task_paging_scope(task); }
+    ~OtherTaskPagingScope() { MM.enter_task_paging_scope(*current); }
+};

Kernel/ProcFileSystem.cpp

@@ -61,11 +61,7 @@ ByteBuffer procfs$pid_vm(const Task& task)
 ByteBuffer procfs$pid_stack(Task& task)
 {
     InterruptDisabler disabler;
-    ASSERT(false);
-    if (current != &task) {
-        MM.unmapRegionsForTask(*current);
-        MM.mapRegionsForTask(task);
-    }
+    OtherTaskPagingScope pagingScope(task);
     struct RecognizedSymbol {
         dword address;
         const KSym* ksym;
@@ -91,10 +87,6 @@ ByteBuffer procfs$pid_stack(Task& task)
         bufptr += ksprintf(bufptr, "%p  %s +%u\n", symbol.address, symbol.ksym->name.characters(), offset);
     }
     buffer.trim(bufptr - (char*)buffer.pointer());
-    if (current != &task) {
-        MM.unmapRegionsForTask(task);
-        MM.mapRegionsForTask(*current);
-    }
     return buffer;
 }
 

Kernel/Task.cpp

@@ -228,11 +228,6 @@ int Task::sys$spawn(const char* path, const char** args)
     return error;
 }
 
-struct KernelPagingScope {
-    KernelPagingScope() { MM.enter_kernel_paging_scope(); }
-    ~KernelPagingScope() { MM.enter_task_paging_scope(*current); }
-};
-
 Task* Task::createUserTask(const String& path, uid_t uid, gid_t gid, pid_t parentPID, int& error, const char** args, TTY* tty)
 {
     auto parts = path.split('/');
@@ -281,7 +276,7 @@ Task* Task::createUserTask(const String& path, uid_t uid, gid_t gid, pid_t paren
     taskEnvironment.append("HOME=/");
 
     InterruptDisabler disabler; // FIXME: Get rid of this, jesus christ. This "critical" section is HUGE.
-    KernelPagingScope kernelScope;
+    KernelPagingScope pagingScope;
     Task* t = new Task(parts.takeLast(), uid, gid, parentPID, Ring3, move(cwd), handle->vnode(), tty);
 
     t->m_arguments = move(taskArguments);