// SPDX-License-Identifier: GPL-2.0
/*
 *  S390 version
 *    Copyright IBM Corp. 1999
 *    Author(s): Hartmut Penner (hp@de.ibm.com)
 *               Ulrich Weigand (uweigand@de.ibm.com)
 *
 *  Derived from "arch/i386/mm/fault.c"
 *    Copyright (C) 1995  Linus Torvalds
 */

#include <linux/kernel_stat.h>
#include <linux/perf_event.h>
#include <linux/signal.h>
#include <linux/sched.h>
#include <linux/sched/debug.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/ptrace.h>
#include <linux/mman.h>
#include <linux/mm.h>
#include <linux/compat.h>
#include <linux/smp.h>
#include <linux/kdebug.h>
#include <linux/init.h>
#include <linux/console.h>
#include <linux/extable.h>
#include <linux/hardirq.h>
#include <linux/kprobes.h>
#include <linux/uaccess.h>
#include <linux/hugetlb.h>
#include <asm/asm-offsets.h>
#include <asm/diag.h>
#include <asm/pgtable.h>
#include <asm/gmap.h>
#include <asm/irq.h>
#include <asm/mmu_context.h>
#include <asm/facility.h>
#include "../kernel/entry.h"

#define __FAIL_ADDR_MASK -4096L
#define __SUBCODE_MASK 0x0600
#define __PF_RES_FIELD 0x8000000000000000ULL

#define VM_FAULT_BADCONTEXT     0x010000
#define VM_FAULT_BADMAP         0x020000
#define VM_FAULT_BADACCESS      0x040000
#define VM_FAULT_SIGNAL         0x080000
#define VM_FAULT_PFAULT         0x100000

enum fault_type {
        KERNEL_FAULT,
        USER_FAULT,
        VDSO_FAULT,
        GMAP_FAULT,
};

static unsigned long store_indication __read_mostly;

static int __init fault_init(void)
{
        if (test_facility(75))
                store_indication = 0xc00;
        return 0;
}
early_initcall(fault_init);

static inline int notify_page_fault(struct pt_regs *regs)
{
        int ret = 0;

        /* kprobe_running() needs smp_processor_id() */
        if (kprobes_built_in() && !user_mode(regs)) {
                preempt_disable();
                if (kprobe_running() && kprobe_fault_handler(regs, 14))
                        ret = 1;
                preempt_enable();
        }
        return ret;
}


/*
 * Unlock any spinlocks which will prevent us from getting the
 * message out.
 */
void bust_spinlocks(int yes)
{
        if (yes) {
                oops_in_progress = 1;
        } else {
                int loglevel_save = console_loglevel;
                console_unblank();
                oops_in_progress = 0;
                /*
                 * OK, the message is on the console.  Now we call printk()
                 * without oops_in_progress set so that printk will give klogd
                 * a poke.  Hold onto your hats...
                 */
                console_loglevel = 15;
                printk(" ");
                console_loglevel = loglevel_save;
        }
}

/*
 * Find out which address space caused the exception.
 */
static inline enum fault_type get_fault_type(struct pt_regs *regs)
{
        unsigned long trans_exc_code;

        trans_exc_code = regs->int_parm_long & 3;
        if (likely(trans_exc_code == 0)) {
                /* primary space exception */
                if (IS_ENABLED(CONFIG_PGSTE) &&
                    test_pt_regs_flag(regs, PIF_GUEST_FAULT))
                        return GMAP_FAULT;
                if (current->thread.mm_segment == USER_DS)
                        return USER_FAULT;
                return KERNEL_FAULT;
        }
        if (trans_exc_code == 2) {
                /* secondary space exception */
                if (current->thread.mm_segment & 1) {
                        if (current->thread.mm_segment == USER_DS_SACF)
                                return USER_FAULT;
                        return KERNEL_FAULT;
                }
                return VDSO_FAULT;
        }
        if (trans_exc_code == 1) {
                /* access register mode, not used in the kernel */
                return USER_FAULT;
        }
        /* home space exception -> access via kernel ASCE */
        return KERNEL_FAULT;
}

static int bad_address(void *p)
{
        unsigned long dummy;

        return probe_kernel_address((unsigned long *)p, dummy);
}

static void dump_pagetable(unsigned long asce, unsigned long address)
{
        unsigned long *table = __va(asce & _ASCE_ORIGIN);

        pr_alert("AS:%016lx ", asce);
        switch (asce & _ASCE_TYPE_MASK) {
        case _ASCE_TYPE_REGION1:
                table += (address & _REGION1_INDEX) >> _REGION1_SHIFT;
                if (bad_address(table))
                        goto bad;
                pr_cont("R1:%016lx ", *table);
                if (*table & _REGION_ENTRY_INVALID)
                        goto out;
                table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
                /* fallthrough */
        case _ASCE_TYPE_REGION2:
                table += (address & _REGION2_INDEX) >> _REGION2_SHIFT;
                if (bad_address(table))
                        goto bad;
                pr_cont("R2:%016lx ", *table);
                if (*table & _REGION_ENTRY_INVALID)
                        goto out;
                table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
                /* fallthrough */
        case _ASCE_TYPE_REGION3:
                table += (address & _REGION3_INDEX) >> _REGION3_SHIFT;
                if (bad_address(table))
                        goto bad;
                pr_cont("R3:%016lx ", *table);
                if (*table & (_REGION_ENTRY_INVALID | _REGION3_ENTRY_LARGE))
                        goto out;
                table = (unsigned long *)(*table & _REGION_ENTRY_ORIGIN);
                /* fallthrough */
        case _ASCE_TYPE_SEGMENT:
                table += (address & _SEGMENT_INDEX) >> _SEGMENT_SHIFT;
                if (bad_address(table))
                        goto bad;
                pr_cont("S:%016lx ", *table);
                if (*table & (_SEGMENT_ENTRY_INVALID | _SEGMENT_ENTRY_LARGE))
                        goto out;
                table = (unsigned long *)(*table & _SEGMENT_ENTRY_ORIGIN);
        }
        table += (address & _PAGE_INDEX) >> _PAGE_SHIFT;
        if (bad_address(table))
                goto bad;
        pr_cont("P:%016lx ", *table);
out:
        pr_cont("\n");
        return;
bad:
        pr_cont("BAD\n");
}

static void dump_fault_info(struct pt_regs *regs)
{
        unsigned long asce;

        pr_alert("Failing address: %016lx TEID: %016lx\n",
                 regs->int_parm_long & __FAIL_ADDR_MASK, regs->int_parm_long);
        pr_alert("Fault in ");
        switch (regs->int_parm_long & 3) {
        case 3:
                pr_cont("home space ");
                break;
        case 2:
                pr_cont("secondary space ");
                break;
        case 1:
                pr_cont("access register ");
                break;
        case 0:
                pr_cont("primary space ");
                break;
        }
        pr_cont("mode while using ");
        switch (get_fault_type(regs)) {
        case USER_FAULT:
                asce = S390_lowcore.user_asce;
                pr_cont("user ");
                break;
        case VDSO_FAULT:
                asce = S390_lowcore.vdso_asce;
                pr_cont("vdso ");
                break;
        case GMAP_FAULT:
                asce = ((struct gmap *) S390_lowcore.gmap)->asce;
                pr_cont("gmap ");
                break;
        case KERNEL_FAULT:
                asce = S390_lowcore.kernel_asce;
                pr_cont("kernel ");
                break;
        }
        pr_cont("ASCE.\n");
        dump_pagetable(asce, regs->int_parm_long & __FAIL_ADDR_MASK);
}

int show_unhandled_signals = 1;

void report_user_fault(struct pt_regs *regs, long signr, int is_mm_fault)
{
        if ((task_pid_nr(current) > 1) && !show_unhandled_signals)
                return;
        if (!unhandled_signal(current, signr))
                return;
        if (!printk_ratelimit())
                return;
        printk(KERN_ALERT "User process fault: interruption code %04x ilc:%d ",
               regs->int_code & 0xffff, regs->int_code >> 17);
        print_vma_addr(KERN_CONT "in ", regs->psw.addr);
        printk(KERN_CONT "\n");
        if (is_mm_fault)
                dump_fault_info(regs);
        show_regs(regs);
}

/*
 * Send SIGSEGV to task.  This is an external routine
 * to keep the stack usage of do_page_fault small.
 */
static noinline void do_sigsegv(struct pt_regs *regs, int si_code)
{
        report_user_fault(regs, SIGSEGV, 1);
        force_sig_fault(SIGSEGV, si_code,
                        (void __user *)(regs->int_parm_long & __FAIL_ADDR_MASK),
                        current);
}

const struct exception_table_entry *s390_search_extables(unsigned long addr)
{
        const struct exception_table_entry *fixup;

        fixup = search_extable(__start_dma_ex_table,
                               __stop_dma_ex_table - __start_dma_ex_table,
                               addr);
        if (!fixup)
                fixup = search_exception_tables(addr);
        return fixup;
}

static noinline void do_no_context(struct pt_regs *regs)
{
        const struct exception_table_entry *fixup;

        /* Are we prepared to handle this kernel fault?  */
        fixup = s390_search_extables(regs->psw.addr);
        if (fixup) {
                regs->psw.addr = extable_fixup(fixup);
                return;
        }

        /*
         * Oops. The kernel tried to access some bad page. We'll have to
         * terminate things with extreme prejudice.
         */
        if (get_fault_type(regs) == KERNEL_FAULT)
                printk(KERN_ALERT "Unable to handle kernel pointer dereference"
                       " in virtual kernel address space\n");
        else
                printk(KERN_ALERT "Unable to handle kernel paging request"
                       " in virtual user address space\n");
        dump_fault_info(regs);
        die(regs, "Oops");
        do_exit(SIGKILL);
}

static noinline void do_low_address(struct pt_regs *regs)
{
        /* Low-address protection hit in kernel mode means
           NULL pointer write access in kernel mode.  */
        if (regs->psw.mask & PSW_MASK_PSTATE) {
                /* Low-address protection hit in user mode 'cannot happen'. */
                die (regs, "Low-address protection");
                do_exit(SIGKILL);
        }

        do_no_context(regs);
}

static noinline void do_sigbus(struct pt_regs *regs)
{
        /*
         * Send a sigbus, regardless of whether we were in kernel
         * or user mode.
         */
        force_sig_fault(SIGBUS, BUS_ADRERR,
                        (void __user *)(regs->int_parm_long & __FAIL_ADDR_MASK),
                        current);
}

static noinline int signal_return(struct pt_regs *regs)
{
        u16 instruction;
        int rc;

        rc = __get_user(instruction, (u16 __user *) regs->psw.addr);
        if (rc)
                return rc;
        if (instruction == 0x0a77) {
                set_pt_regs_flag(regs, PIF_SYSCALL);
                regs->int_code = 0x00040077;
                return 0;
        } else if (instruction == 0x0aad) {
                set_pt_regs_flag(regs, PIF_SYSCALL);
                regs->int_code = 0x000400ad;
                return 0;
        }
        return -EACCES;
}

static noinline void do_fault_error(struct pt_regs *regs, int access,
                                        vm_fault_t fault)
{
        int si_code;

        switch (fault) {
        case VM_FAULT_BADACCESS:
                if (access == VM_EXEC && signal_return(regs) == 0)
                        break;
        case VM_FAULT_BADMAP:
                /* Bad memory access. Check if it is kernel or user space. */
                if (user_mode(regs)) {
                        /* User mode accesses just cause a SIGSEGV */
                        si_code = (fault == VM_FAULT_BADMAP) ?
                                SEGV_MAPERR : SEGV_ACCERR;
                        do_sigsegv(regs, si_code);
                        break;
                }
        case VM_FAULT_BADCONTEXT:
        case VM_FAULT_PFAULT:
                do_no_context(regs);
                break;
        case VM_FAULT_SIGNAL:
                if (!user_mode(regs))
                        do_no_context(regs);
                break;
        default: /* fault & VM_FAULT_ERROR */
                if (fault & VM_FAULT_OOM) {
                        if (!user_mode(regs))
                                do_no_context(regs);
                        else
                                pagefault_out_of_memory();
                } else if (fault & VM_FAULT_SIGSEGV) {
                        /* Kernel mode? Handle exceptions or die */
                        if (!user_mode(regs))
                                do_no_context(regs);
                        else
                                do_sigsegv(regs, SEGV_MAPERR);
                } else if (fault & VM_FAULT_SIGBUS) {
                        /* Kernel mode? Handle exceptions or die */
                        if (!user_mode(regs))
                                do_no_context(regs);
                        else
                                do_sigbus(regs);
                } else
                        BUG();
                break;
        }
}

/*
 * This routine handles page faults.  It determines the address,
 * and the problem, and then passes it off to one of the appropriate
 * routines.
 *
 * interruption code (int_code):
 *   04       Protection           ->  Write-Protection  (suprression)
 *   10       Segment translation  ->  Not present       (nullification)
 *   11       Page translation     ->  Not present       (nullification)
 *   3b       Region third trans.  ->  Not present       (nullification)
 */
static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
{
        struct gmap *gmap;
        struct task_struct *tsk;
        struct mm_struct *mm;
        struct vm_area_struct *vma;
        enum fault_type type;
        unsigned long trans_exc_code;
        unsigned long address;
        unsigned int flags;
        vm_fault_t fault;

        tsk = current;
        /*
         * The instruction that caused the program check has
         * been nullified. Don't signal single step via SIGTRAP.
         */
        clear_pt_regs_flag(regs, PIF_PER_TRAP);

        if (notify_page_fault(regs))
                return 0;

        mm = tsk->mm;
        trans_exc_code = regs->int_parm_long;

        /*
         * Verify that the fault happened in user space, that
         * we are not in an interrupt and that there is a 
         * user context.
         */
        fault = VM_FAULT_BADCONTEXT;
        type = get_fault_type(regs);
        switch (type) {
        case KERNEL_FAULT:
                goto out;
        case VDSO_FAULT:
                fault = VM_FAULT_BADMAP;
                goto out;
        case USER_FAULT:
        case GMAP_FAULT:
                if (faulthandler_disabled() || !mm)
                        goto out;
                break;
        }

        address = trans_exc_code & __FAIL_ADDR_MASK;
        perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
        flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
        if (user_mode(regs))
                flags |= FAULT_FLAG_USER;
        if (access == VM_WRITE || (trans_exc_code & store_indication) == 0x400)
                flags |= FAULT_FLAG_WRITE;
        down_read(&mm->mmap_sem);

        gmap = NULL;
        if (IS_ENABLED(CONFIG_PGSTE) && type == GMAP_FAULT) {
                gmap = (struct gmap *) S390_lowcore.gmap;
                current->thread.gmap_addr = address;
                current->thread.gmap_write_flag = !!(flags & FAULT_FLAG_WRITE);
                current->thread.gmap_int_code = regs->int_code & 0xffff;
                address = __gmap_translate(gmap, address);
                if (address == -EFAULT) {
                        fault = VM_FAULT_BADMAP;
                        goto out_up;
                }
                if (gmap->pfault_enabled)
                        flags |= FAULT_FLAG_RETRY_NOWAIT;
        }

retry:
        fault = VM_FAULT_BADMAP;
        vma = find_vma(mm, address);
        if (!vma)
                goto out_up;

        if (unlikely(vma->vm_start > address)) {
                if (!(vma->vm_flags & VM_GROWSDOWN))
                        goto out_up;
                if (expand_stack(vma, address))
                        goto out_up;
        }

        /*
         * Ok, we have a good vm_area for this memory access, so
         * we can handle it..
         */
        fault = VM_FAULT_BADACCESS;
        if (unlikely(!(vma->vm_flags & access)))
                goto out_up;

        if (is_vm_hugetlb_page(vma))
                address &= HPAGE_MASK;
        /*
         * If for any reason at all we couldn't handle the fault,
         * make sure we exit gracefully rather than endlessly redo
         * the fault.
         */
        fault = handle_mm_fault(vma, address, flags);
        /* No reason to continue if interrupted by SIGKILL. */
        if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
                fault = VM_FAULT_SIGNAL;
                if (flags & FAULT_FLAG_RETRY_NOWAIT)
                        goto out_up;
                goto out;
        }
        if (unlikely(fault & VM_FAULT_ERROR))
                goto out_up;

        /*
         * Major/minor page fault accounting is only done on the
         * initial attempt. If we go through a retry, it is extremely
         * likely that the page will be found in page cache at that point.
         */
        if (flags & FAULT_FLAG_ALLOW_RETRY) {
                if (fault & VM_FAULT_MAJOR) {
                        tsk->maj_flt++;
                        perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
                                      regs, address);
                } else {
                        tsk->min_flt++;
                        perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
                                      regs, address);
                }
                if (fault & VM_FAULT_RETRY) {
                        if (IS_ENABLED(CONFIG_PGSTE) && gmap &&
                            (flags & FAULT_FLAG_RETRY_NOWAIT)) {
                                /* FAULT_FLAG_RETRY_NOWAIT has been set,
                                 * mmap_sem has not been released */
                                current->thread.gmap_pfault = 1;
                                fault = VM_FAULT_PFAULT;
                                goto out_up;
                        }
                        /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
                         * of starvation. */
                        flags &= ~(FAULT_FLAG_ALLOW_RETRY |
                                   FAULT_FLAG_RETRY_NOWAIT);
                        flags |= FAULT_FLAG_TRIED;
                        down_read(&mm->mmap_sem);
                        goto retry;
                }
        }
        if (IS_ENABLED(CONFIG_PGSTE) && gmap) {
                address =  __gmap_link(gmap, current->thread.gmap_addr,
                                       address);
                if (address == -EFAULT) {
                        fault = VM_FAULT_BADMAP;
                        goto out_up;
                }
                if (address == -ENOMEM) {
                        fault = VM_FAULT_OOM;
                        goto out_up;
                }
        }
        fault = 0;
out_up:
        up_read(&mm->mmap_sem);
out:
        return fault;
}

void do_protection_exception(struct pt_regs *regs)
{
        unsigned long trans_exc_code;
        int access;
        vm_fault_t fault;

        trans_exc_code = regs->int_parm_long;
        /*
         * Protection exceptions are suppressing, decrement psw address.
         * The exception to this rule are aborted transactions, for these
         * the PSW already points to the correct location.
         */
        if (!(regs->int_code & 0x200))
                regs->psw.addr = __rewind_psw(regs->psw, regs->int_code >> 16);
        /*
         * Check for low-address protection.  This needs to be treated
         * as a special case because the translation exception code
         * field is not guaranteed to contain valid data in this case.
         */
        if (unlikely(!(trans_exc_code & 4))) {
                do_low_address(regs);
                return;
        }
        if (unlikely(MACHINE_HAS_NX && (trans_exc_code & 0x80))) {
                regs->int_parm_long = (trans_exc_code & ~PAGE_MASK) |
                                        (regs->psw.addr & PAGE_MASK);
                access = VM_EXEC;
                fault = VM_FAULT_BADACCESS;
        } else {
                access = VM_WRITE;
                fault = do_exception(regs, access);
        }
        if (unlikely(fault))
                do_fault_error(regs, access, fault);
}
NOKPROBE_SYMBOL(do_protection_exception);

void do_dat_exception(struct pt_regs *regs)
{
        int access;
        vm_fault_t fault;

        access = VM_READ | VM_EXEC | VM_WRITE;
        fault = do_exception(regs, access);
        if (unlikely(fault))
                do_fault_error(regs, access, fault);
}
NOKPROBE_SYMBOL(do_dat_exception);

#ifdef CONFIG_PFAULT 
/*
 * 'pfault' pseudo page faults routines.
 */
static int pfault_disable;

static int __init nopfault(char *str)
{
        pfault_disable = 1;
        return 1;
}

__setup("nopfault", nopfault);

struct pfault_refbk {
        u16 refdiagc;
        u16 reffcode;
        u16 refdwlen;
        u16 refversn;
        u64 refgaddr;
        u64 refselmk;
        u64 refcmpmk;
        u64 reserved;
} __attribute__ ((packed, aligned(8)));

int pfault_init(void)
{
        struct pfault_refbk refbk = {
                .refdiagc = 0x258,
                .reffcode = 0,
                .refdwlen = 5,
                .refversn = 2,
                .refgaddr = __LC_LPP,
                .refselmk = 1ULL << 48,
                .refcmpmk = 1ULL << 48,
                .reserved = __PF_RES_FIELD };
        int rc;

        if (pfault_disable)
                return -1;
        diag_stat_inc(DIAG_STAT_X258);
        asm volatile(
                "       diag    %1,%0,0x258\n"
                "0:     j       2f\n"
                "1:     la      %0,8\n"
                "2:\n"
                EX_TABLE(0b,1b)
                : "=d" (rc) : "a" (&refbk), "m" (refbk) : "cc");
        return rc;
}

void pfault_fini(void)
{
        struct pfault_refbk refbk = {
                .refdiagc = 0x258,
                .reffcode = 1,
                .refdwlen = 5,
                .refversn = 2,
        };

        if (pfault_disable)
                return;
        diag_stat_inc(DIAG_STAT_X258);
        asm volatile(
                "       diag    %0,0,0x258\n"
                "0:     nopr    %%r7\n"
                EX_TABLE(0b,0b)
                : : "a" (&refbk), "m" (refbk) : "cc");
}

static DEFINE_SPINLOCK(pfault_lock);
static LIST_HEAD(pfault_list);

#define PF_COMPLETE     0x0080

/*
 * The mechanism of our pfault code: if Linux is running as guest, runs a user
 * space process and the user space process accesses a page that the host has
 * paged out we get a pfault interrupt.
 *
 * This allows us, within the guest, to schedule a different process. Without
 * this mechanism the host would have to suspend the whole virtual cpu until
 * the page has been paged in.
 *
 * So when we get such an interrupt then we set the state of the current task
 * to uninterruptible and also set the need_resched flag. Both happens within
 * interrupt context(!). If we later on want to return to user space we
 * recognize the need_resched flag and then call schedule().  It's not very
 * obvious how this works...
 *
 * Of course we have a lot of additional fun with the completion interrupt (->
 * host signals that a page of a process has been paged in and the process can
 * continue to run). This interrupt can arrive on any cpu and, since we have
 * virtual cpus, actually appear before the interrupt that signals that a page
 * is missing.
 */
static void pfault_interrupt(struct ext_code ext_code,
                             unsigned int param32, unsigned long param64)
{
        struct task_struct *tsk;
        __u16 subcode;
        pid_t pid;

        /*
         * Get the external interruption subcode & pfault initial/completion
         * signal bit. VM stores this in the 'cpu address' field associated
         * with the external interrupt.
         */
        subcode = ext_code.subcode;
        if ((subcode & 0xff00) != __SUBCODE_MASK)
                return;
        inc_irq_stat(IRQEXT_PFL);
        /* Get the token (= pid of the affected task). */
        pid = param64 & LPP_PID_MASK;
        rcu_read_lock();
        tsk = find_task_by_pid_ns(pid, &init_pid_ns);
        if (tsk)
                get_task_struct(tsk);
        rcu_read_unlock();
        if (!tsk)
                return;
        spin_lock(&pfault_lock);
        if (subcode & PF_COMPLETE) {
                /* signal bit is set -> a page has been swapped in by VM */
                if (tsk->thread.pfault_wait == 1) {
                        /* Initial interrupt was faster than the completion
                         * interrupt. pfault_wait is valid. Set pfault_wait
                         * back to zero and wake up the process. This can
                         * safely be done because the task is still sleeping
                         * and can't produce new pfaults. */
                        tsk->thread.pfault_wait = 0;
                        list_del(&tsk->thread.list);
                        wake_up_process(tsk);
                        put_task_struct(tsk);
                } else {
                        /* Completion interrupt was faster than initial
                         * interrupt. Set pfault_wait to -1 so the initial
                         * interrupt doesn't put the task to sleep.
                         * If the task is not running, ignore the completion
                         * interrupt since it must be a leftover of a PFAULT
                         * CANCEL operation which didn't remove all pending
                         * completion interrupts. */
                        if (tsk->state == TASK_RUNNING)
                                tsk->thread.pfault_wait = -1;
                }
        } else {
                /* signal bit not set -> a real page is missing. */
                if (WARN_ON_ONCE(tsk != current))
                        goto out;
                if (tsk->thread.pfault_wait == 1) {
                        /* Already on the list with a reference: put to sleep */
                        goto block;
                } else if (tsk->thread.pfault_wait == -1) {
                        /* Completion interrupt was faster than the initial
                         * interrupt (pfault_wait == -1). Set pfault_wait
                         * back to zero and exit. */
                        tsk->thread.pfault_wait = 0;
                } else {
                        /* Initial interrupt arrived before completion
                         * interrupt. Let the task sleep.
                         * An extra task reference is needed since a different
                         * cpu may set the task state to TASK_RUNNING again
                         * before the scheduler is reached. */
                        get_task_struct(tsk);
                        tsk->thread.pfault_wait = 1;
                        list_add(&tsk->thread.list, &pfault_list);
block:
                        /* Since this must be a userspace fault, there
                         * is no kernel task state to trample. Rely on the
                         * return to userspace schedule() to block. */
                        __set_current_state(TASK_UNINTERRUPTIBLE);
                        set_tsk_need_resched(tsk);
                        set_preempt_need_resched();
                }
        }
out:
        spin_unlock(&pfault_lock);
        put_task_struct(tsk);
}

static int pfault_cpu_dead(unsigned int cpu)
{
        struct thread_struct *thread, *next;
        struct task_struct *tsk;

        spin_lock_irq(&pfault_lock);
        list_for_each_entry_safe(thread, next, &pfault_list, list) {
                thread->pfault_wait = 0;
                list_del(&thread->list);
                tsk = container_of(thread, struct task_struct, thread);
                wake_up_process(tsk);
                put_task_struct(tsk);
        }
        spin_unlock_irq(&pfault_lock);
        return 0;
}

static int __init pfault_irq_init(void)
{
        int rc;

        rc = register_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
        if (rc)
                goto out_extint;
        rc = pfault_init() == 0 ? 0 : -EOPNOTSUPP;
        if (rc)
                goto out_pfault;
        irq_subclass_register(IRQ_SUBCLASS_SERVICE_SIGNAL);
        cpuhp_setup_state_nocalls(CPUHP_S390_PFAULT_DEAD, "s390/pfault:dead",
                                  NULL, pfault_cpu_dead);
        return 0;

out_pfault:
        unregister_external_irq(EXT_IRQ_CP_SERVICE, pfault_interrupt);
out_extint:
        pfault_disable = 1;
        return rc;
}
early_initcall(pfault_irq_init);

#endif /* CONFIG_PFAULT */