um: Stop tracking stub's PID via userspace_pid[]

The PID of the stub process can be obtained from current_mm_id().
There is no need to track it via userspace_pid[]. Stop doing that
to simplify the code.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250711065021.2535362-4-tiwei.bie@linux.dev
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/include/asm/mmu_context.h

@@ -16,23 +16,16 @@
 #define activate_mm activate_mm
 static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 {
-	/*
-	 * This is called by fs/exec.c and sys_unshare()
-	 * when the new ->mm is used for the first time.
-	 */
-	__switch_mm(&new->context.id);
 }
 
 static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, 
 			     struct task_struct *tsk)
 {
 	unsigned cpu = smp_processor_id();
 
-	if(prev != next){
+	if (prev != next) {
 		cpumask_clear_cpu(cpu, mm_cpumask(prev));
 		cpumask_set_cpu(cpu, mm_cpumask(next));
-		if(next != &init_mm)
-			__switch_mm(&next->context.id);
 	}
 }
 

arch/um/include/shared/skas/mm_id.h

@@ -19,8 +19,6 @@ struct mm_id {
 	int syscall_fd_map[STUB_MAX_FDS];
 };
 
-void __switch_mm(struct mm_id *mm_idp);
-
 void notify_mm_kill(int pid);
 
 #endif

arch/um/include/shared/skas/skas.h

@@ -9,7 +9,6 @@
 #include <sysdep/ptrace.h>
 
 extern int using_seccomp;
-extern int userspace_pid[];
 
 extern void new_thread_handler(void);
 extern void handle_syscall(struct uml_pt_regs *regs);

arch/um/kernel/exec.c

@@ -26,8 +26,6 @@ void flush_thread(void)
 
 	get_safe_registers(current_pt_regs()->regs.gp,
 			   current_pt_regs()->regs.fp);
-
-	__switch_mm(&current->mm->context.id);
 }
 
 void start_thread(struct pt_regs *regs, unsigned long eip, unsigned long esp)

arch/um/kernel/skas/process.c

@@ -26,8 +26,6 @@ static int __init start_kernel_proc(void *unused)
 	return 0;
 }
 
-extern int userspace_pid[];
-
 static char cpu0_irqstack[THREAD_SIZE] __aligned(THREAD_SIZE);
 
 int __init start_uml(void)

arch/um/os-Linux/skas/process.c

@@ -434,7 +434,6 @@ static int __init init_stub_exe_fd(void)
 __initcall(init_stub_exe_fd);
 
 int using_seccomp;
-int userspace_pid[NR_CPUS];
 
 /**
  * start_userspace() - prepare a new userspace process
@@ -553,7 +552,7 @@ extern unsigned long tt_extra_sched_jiffies;
 
 void userspace(struct uml_pt_regs *regs)
 {
-	int err, status, op, pid = userspace_pid[0];
+	int err, status, op;
 	siginfo_t si_ptrace;
 	siginfo_t *si;
 	int sig;
@@ -562,6 +561,8 @@ void userspace(struct uml_pt_regs *regs)
 	interrupt_end();
 
 	while (1) {
+		struct mm_id *mm_id = current_mm_id();
+
 		/*
 		 * When we are in time-travel mode, userspace can theoretically
 		 * do a *lot* of work without being scheduled. The problem with
@@ -590,7 +591,6 @@ void userspace(struct uml_pt_regs *regs)
 		current_mm_sync();
 
 		if (using_seccomp) {
-			struct mm_id *mm_id = current_mm_id();
 			struct stub_data *proc_data = (void *) mm_id->stack;
 
 			err = set_stub_state(regs, proc_data, singlestepping());
@@ -644,8 +644,10 @@ void userspace(struct uml_pt_regs *regs)
 				GET_FAULTINFO_FROM_MC(regs->faultinfo, mcontext);
 			}
 		} else {
+			int pid = mm_id->pid;
+
 			/* Flush out any pending syscalls */
-			err = syscall_stub_flush(current_mm_id());
+			err = syscall_stub_flush(mm_id);
 			if (err) {
 				if (err == -ENOMEM)
 					report_enomem();
@@ -776,7 +778,6 @@ void userspace(struct uml_pt_regs *regs)
 				       __func__, sig);
 				fatal_sigsegv();
 			}
-			pid = userspace_pid[0];
 			interrupt_end();
 
 			/* Avoid -ERESTARTSYS handling in host */
@@ -901,8 +902,3 @@ void reboot_skas(void)
 	block_signals_trace();
 	UML_LONGJMP(&initial_jmpbuf, noreboot ? INIT_JMP_HALT : INIT_JMP_REBOOT);
 }
-
-void __switch_mm(struct mm_id *mm_idp)
-{
-	userspace_pid[0] = mm_idp->pid;
-}

arch/x86/um/tls_32.c

@@ -186,7 +186,7 @@ int arch_switch_tls(struct task_struct *to)
 	/*
 	 * We have no need whatsoever to switch TLS for kernel threads; beyond
 	 * that, that would also result in us calling os_set_thread_area with
-	 * userspace_pid[cpu] == 0, which gives an error.
+	 * task->mm == NULL, which would cause a crash.
 	 */
 	if (likely(to->mm))
 		return load_TLS(O_FORCE, to);