// SPDX-License-Identifier: GPL-2.0
#include <linux/debugfs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/memblock.h>
#include <linux/stacktrace.h>
#include <linux/page_owner.h>
#include <linux/jump_label.h>
#include <linux/migrate.h>
#include <linux/stackdepot.h>
#include <linux/seq_file.h>
#include <linux/memcontrol.h>
#include <linux/sched/clock.h>

#include "internal.h"

/*
 * TODO: teach PAGE_OWNER_STACK_DEPTH (__dump_page_owner and save_stack)
 * to use off stack temporal storage
 */
#define PAGE_OWNER_STACK_DEPTH (16)

struct page_owner {
        unsigned short order;
        short last_migrate_reason;
        gfp_t gfp_mask;
        depot_stack_handle_t handle;
        depot_stack_handle_t free_handle;
        u64 ts_nsec;
        u64 free_ts_nsec;
        char comm[TASK_COMM_LEN];
        pid_t pid;
        pid_t tgid;
        pid_t free_pid;
        pid_t free_tgid;
};

struct stack {
        struct stack_record *stack_record;
        struct stack *next;
};
static struct stack dummy_stack;
static struct stack failure_stack;
static struct stack *stack_list;
static DEFINE_SPINLOCK(stack_list_lock);

static bool page_owner_enabled __initdata;
DEFINE_STATIC_KEY_FALSE(page_owner_inited);

static depot_stack_handle_t dummy_handle;
static depot_stack_handle_t failure_handle;
static depot_stack_handle_t early_handle;

static void init_early_allocated_pages(void);

static inline void set_current_in_page_owner(void)
{
        /*
         * Avoid recursion.
         *
         * We might need to allocate more memory from page_owner code, so make
         * sure to signal it in order to avoid recursion.
         */
        current->in_page_owner = 1;
}

static inline void unset_current_in_page_owner(void)
{
        current->in_page_owner = 0;
}

static int __init early_page_owner_param(char *buf)
{
        int ret = kstrtobool(buf, &page_owner_enabled);

        if (page_owner_enabled)
                stack_depot_request_early_init();

        return ret;
}
early_param("page_owner", early_page_owner_param);

static __init bool need_page_owner(void)
{
        return page_owner_enabled;
}

static __always_inline depot_stack_handle_t create_dummy_stack(void)
{
        unsigned long entries[4];
        unsigned int nr_entries;

        nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
        return stack_depot_save(entries, nr_entries, GFP_KERNEL);
}

static noinline void register_dummy_stack(void)
{
        dummy_handle = create_dummy_stack();
}

static noinline void register_failure_stack(void)
{
        failure_handle = create_dummy_stack();
}

static noinline void register_early_stack(void)
{
        early_handle = create_dummy_stack();
}

static __init void init_page_owner(void)
{
        if (!page_owner_enabled)
                return;

        register_dummy_stack();
        register_failure_stack();
        register_early_stack();
        init_early_allocated_pages();
        /* Initialize dummy and failure stacks and link them to stack_list */
        dummy_stack.stack_record = __stack_depot_get_stack_record(dummy_handle);
        failure_stack.stack_record = __stack_depot_get_stack_record(failure_handle);
        if (dummy_stack.stack_record)
                refcount_set(&dummy_stack.stack_record->count, 1);
        if (failure_stack.stack_record)
                refcount_set(&failure_stack.stack_record->count, 1);
        dummy_stack.next = &failure_stack;
        stack_list = &dummy_stack;
        static_branch_enable(&page_owner_inited);
}

struct page_ext_operations page_owner_ops = {
        .size = sizeof(struct page_owner),
        .need = need_page_owner,
        .init = init_page_owner,
        .need_shared_flags = true,
};

static inline struct page_owner *get_page_owner(struct page_ext *page_ext)
{
        return page_ext_data(page_ext, &page_owner_ops);
}

static noinline depot_stack_handle_t save_stack(gfp_t flags)
{
        unsigned long entries[PAGE_OWNER_STACK_DEPTH];
        depot_stack_handle_t handle;
        unsigned int nr_entries;

        if (current->in_page_owner)
                return dummy_handle;

        set_current_in_page_owner();
        nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 2);
        handle = stack_depot_save(entries, nr_entries, flags);
        if (!handle)
                handle = failure_handle;
        unset_current_in_page_owner();

        return handle;
}

static void add_stack_record_to_list(struct stack_record *stack_record,
                                     gfp_t gfp_mask)
{
        unsigned long flags;
        struct stack *stack;

        set_current_in_page_owner();
        stack = kmalloc(sizeof(*stack), gfp_nested_mask(gfp_mask));
        if (!stack) {
                unset_current_in_page_owner();
                return;
        }
        unset_current_in_page_owner();

        stack->stack_record = stack_record;
        stack->next = NULL;

        spin_lock_irqsave(&stack_list_lock, flags);
        stack->next = stack_list;
        /*
         * This pairs with smp_load_acquire() from function
         * stack_start(). This guarantees that stack_start()
         * will see an updated stack_list before starting to
         * traverse the list.
         */
        smp_store_release(&stack_list, stack);
        spin_unlock_irqrestore(&stack_list_lock, flags);
}

static void inc_stack_record_count(depot_stack_handle_t handle, gfp_t gfp_mask,
                                   int nr_base_pages)
{
        struct stack_record *stack_record = __stack_depot_get_stack_record(handle);

        if (!stack_record)
                return;

        /*
         * New stack_record's that do not use STACK_DEPOT_FLAG_GET start
         * with REFCOUNT_SATURATED to catch spurious increments of their
         * refcount.
         * Since we do not use STACK_DEPOT_FLAG_GET API, let us
         * set a refcount of 1 ourselves.
         */
        if (refcount_read(&stack_record->count) == REFCOUNT_SATURATED) {
                int old = REFCOUNT_SATURATED;

                if (atomic_try_cmpxchg_relaxed(&stack_record->count.refs, &old, 1))
                        /* Add the new stack_record to our list */
                        add_stack_record_to_list(stack_record, gfp_mask);
        }
        refcount_add(nr_base_pages, &stack_record->count);
}

static void dec_stack_record_count(depot_stack_handle_t handle,
                                   int nr_base_pages)
{
        struct stack_record *stack_record = __stack_depot_get_stack_record(handle);

        if (!stack_record)
                return;

        if (refcount_sub_and_test(nr_base_pages, &stack_record->count))
                pr_warn("%s: refcount went to 0 for %u handle\n", __func__,
                        handle);
}

static inline void __update_page_owner_handle(struct page *page,
                                              depot_stack_handle_t handle,
                                              unsigned short order,
                                              gfp_t gfp_mask,
                                              short last_migrate_reason, u64 ts_nsec,
                                              pid_t pid, pid_t tgid, char *comm)
{
        struct page_ext_iter iter;
        struct page_ext *page_ext;
        struct page_owner *page_owner;

        rcu_read_lock();
        for_each_page_ext(page, 1 << order, page_ext, iter) {
                page_owner = get_page_owner(page_ext);
                page_owner->handle = handle;
                page_owner->order = order;
                page_owner->gfp_mask = gfp_mask;
                page_owner->last_migrate_reason = last_migrate_reason;
                page_owner->pid = pid;
                page_owner->tgid = tgid;
                page_owner->ts_nsec = ts_nsec;
                strscpy(page_owner->comm, comm,
                        sizeof(page_owner->comm));
                __set_bit(PAGE_EXT_OWNER, &page_ext->flags);
                __set_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
        }
        rcu_read_unlock();
}

static inline void __update_page_owner_free_handle(struct page *page,
                                                   depot_stack_handle_t handle,
                                                   unsigned short order,
                                                   pid_t pid, pid_t tgid,
                                                   u64 free_ts_nsec)
{
        struct page_ext_iter iter;
        struct page_ext *page_ext;
        struct page_owner *page_owner;

        rcu_read_lock();
        for_each_page_ext(page, 1 << order, page_ext, iter) {
                page_owner = get_page_owner(page_ext);
                /* Only __reset_page_owner() wants to clear the bit */
                if (handle) {
                        __clear_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
                        page_owner->free_handle = handle;
                }
                page_owner->free_ts_nsec = free_ts_nsec;
                page_owner->free_pid = current->pid;
                page_owner->free_tgid = current->tgid;
        }
        rcu_read_unlock();
}

void __reset_page_owner(struct page *page, unsigned short order)
{
        struct page_ext *page_ext;
        depot_stack_handle_t handle;
        depot_stack_handle_t alloc_handle;
        struct page_owner *page_owner;
        u64 free_ts_nsec = local_clock();

        page_ext = page_ext_get(page);
        if (unlikely(!page_ext))
                return;

        page_owner = get_page_owner(page_ext);
        alloc_handle = page_owner->handle;
        page_ext_put(page_ext);

        /*
         * Do not specify GFP_NOWAIT to make gfpflags_allow_spinning() == false
         * to prevent issues in stack_depot_save().
         * This is similar to alloc_pages_nolock() gfp flags, but only used
         * to signal stack_depot to avoid spin_locks.
         */
        handle = save_stack(__GFP_NOWARN);
        __update_page_owner_free_handle(page, handle, order, current->pid,
                                        current->tgid, free_ts_nsec);

        if (alloc_handle != early_handle)
                /*
                 * early_handle is being set as a handle for all those
                 * early allocated pages. See init_pages_in_zone().
                 * Since their refcount is not being incremented because
                 * the machinery is not ready yet, we cannot decrement
                 * their refcount either.
                 */
                dec_stack_record_count(alloc_handle, 1 << order);
}

noinline void __set_page_owner(struct page *page, unsigned short order,
                                        gfp_t gfp_mask)
{
        u64 ts_nsec = local_clock();
        depot_stack_handle_t handle;

        handle = save_stack(gfp_mask);
        __update_page_owner_handle(page, handle, order, gfp_mask, -1,
                                   ts_nsec, current->pid, current->tgid,
                                   current->comm);
        inc_stack_record_count(handle, gfp_mask, 1 << order);
}

void __folio_set_owner_migrate_reason(struct folio *folio, int reason)
{
        struct page_ext *page_ext = page_ext_get(&folio->page);
        struct page_owner *page_owner;

        if (unlikely(!page_ext))
                return;

        page_owner = get_page_owner(page_ext);
        page_owner->last_migrate_reason = reason;
        page_ext_put(page_ext);
}

void __split_page_owner(struct page *page, int old_order, int new_order)
{
        struct page_ext_iter iter;
        struct page_ext *page_ext;
        struct page_owner *page_owner;

        rcu_read_lock();
        for_each_page_ext(page, 1 << old_order, page_ext, iter) {
                page_owner = get_page_owner(page_ext);
                page_owner->order = new_order;
        }
        rcu_read_unlock();
}

void __folio_copy_owner(struct folio *newfolio, struct folio *old)
{
        struct page_ext *page_ext;
        struct page_ext_iter iter;
        struct page_owner *old_page_owner;
        struct page_owner *new_page_owner;
        depot_stack_handle_t migrate_handle;

        page_ext = page_ext_get(&old->page);
        if (unlikely(!page_ext))
                return;

        old_page_owner = get_page_owner(page_ext);
        page_ext_put(page_ext);

        page_ext = page_ext_get(&newfolio->page);
        if (unlikely(!page_ext))
                return;

        new_page_owner = get_page_owner(page_ext);
        page_ext_put(page_ext);

        migrate_handle = new_page_owner->handle;
        __update_page_owner_handle(&newfolio->page, old_page_owner->handle,
                                   old_page_owner->order, old_page_owner->gfp_mask,
                                   old_page_owner->last_migrate_reason,
                                   old_page_owner->ts_nsec, old_page_owner->pid,
                                   old_page_owner->tgid, old_page_owner->comm);
        /*
         * Do not proactively clear PAGE_EXT_OWNER{_ALLOCATED} bits as the folio
         * will be freed after migration. Keep them until then as they may be
         * useful.
         */
        __update_page_owner_free_handle(&newfolio->page, 0, old_page_owner->order,
                                        old_page_owner->free_pid,
                                        old_page_owner->free_tgid,
                                        old_page_owner->free_ts_nsec);
        /*
         * We linked the original stack to the new folio, we need to do the same
         * for the new one and the old folio otherwise there will be an imbalance
         * when subtracting those pages from the stack.
         */
        rcu_read_lock();
        for_each_page_ext(&old->page, 1 << new_page_owner->order, page_ext, iter) {
                old_page_owner = get_page_owner(page_ext);
                old_page_owner->handle = migrate_handle;
        }
        rcu_read_unlock();
}

void pagetypeinfo_showmixedcount_print(struct seq_file *m,
                                       pg_data_t *pgdat, struct zone *zone)
{
        struct page *page;
        struct page_ext *page_ext;
        struct page_owner *page_owner;
        unsigned long pfn, block_end_pfn;
        unsigned long end_pfn = zone_end_pfn(zone);
        unsigned long count[MIGRATE_TYPES] = { 0, };
        int pageblock_mt, page_mt;
        int i;

        /* Scan block by block. First and last block may be incomplete */
        pfn = zone->zone_start_pfn;

        /*
         * Walk the zone in pageblock_nr_pages steps. If a page block spans
         * a zone boundary, it will be double counted between zones. This does
         * not matter as the mixed block count will still be correct
         */
        for (; pfn < end_pfn; ) {
                page = pfn_to_online_page(pfn);
                if (!page) {
                        pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES);
                        continue;
                }

                block_end_pfn = pageblock_end_pfn(pfn);
                block_end_pfn = min(block_end_pfn, end_pfn);

                pageblock_mt = get_pageblock_migratetype(page);

                for (; pfn < block_end_pfn; pfn++) {
                        /* The pageblock is online, no need to recheck. */
                        page = pfn_to_page(pfn);

                        if (page_zone(page) != zone)
                                continue;

                        if (PageBuddy(page)) {
                                unsigned long freepage_order;

                                freepage_order = buddy_order_unsafe(page);
                                if (freepage_order <= MAX_PAGE_ORDER)
                                        pfn += (1UL << freepage_order) - 1;
                                continue;
                        }

                        if (PageReserved(page))
                                continue;

                        page_ext = page_ext_get(page);
                        if (unlikely(!page_ext))
                                continue;

                        if (!test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
                                goto ext_put_continue;

                        page_owner = get_page_owner(page_ext);
                        page_mt = gfp_migratetype(page_owner->gfp_mask);
                        if (pageblock_mt != page_mt) {
                                if (is_migrate_cma(pageblock_mt))
                                        count[MIGRATE_MOVABLE]++;
                                else
                                        count[pageblock_mt]++;

                                pfn = block_end_pfn;
                                page_ext_put(page_ext);
                                break;
                        }
                        pfn += (1UL << page_owner->order) - 1;
ext_put_continue:
                        page_ext_put(page_ext);
                }
        }

        /* Print counts */
        seq_printf(m, "Node %d, zone %8s ", pgdat->node_id, zone->name);
        for (i = 0; i < MIGRATE_TYPES; i++)
                seq_printf(m, "%12lu ", count[i]);
        seq_putc(m, '\n');
}

/*
 * Looking for memcg information and print it out
 */
static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,
                                         struct page *page)
{
#ifdef CONFIG_MEMCG
        unsigned long memcg_data;
        struct mem_cgroup *memcg;
        bool online;
        char name[80];

        rcu_read_lock();
        memcg_data = READ_ONCE(page->memcg_data);
        if (!memcg_data || PageTail(page))
                goto out_unlock;

        if (memcg_data & MEMCG_DATA_OBJEXTS)
                ret += scnprintf(kbuf + ret, count - ret,
                                "Slab cache page\n");

        memcg = page_memcg_check(page);
        if (!memcg)
                goto out_unlock;

        online = (memcg->css.flags & CSS_ONLINE);
        cgroup_name(memcg->css.cgroup, name, sizeof(name));
        ret += scnprintf(kbuf + ret, count - ret,
                        "Charged %sto %smemcg %s\n",
                        PageMemcgKmem(page) ? "(via objcg) " : "",
                        online ? "" : "offline ",
                        name);
out_unlock:
        rcu_read_unlock();
#endif /* CONFIG_MEMCG */

        return ret;
}

static ssize_t
print_page_owner(char __user *buf, size_t count, unsigned long pfn,
                struct page *page, struct page_owner *page_owner,
                depot_stack_handle_t handle)
{
        int ret, pageblock_mt, page_mt;
        char *kbuf;

        count = min_t(size_t, count, PAGE_SIZE);
        kbuf = kmalloc(count, GFP_KERNEL);
        if (!kbuf)
                return -ENOMEM;

        ret = scnprintf(kbuf, count,
                        "Page allocated via order %u, mask %#x(%pGg), pid %d, tgid %d (%s), ts %llu ns\n",
                        page_owner->order, page_owner->gfp_mask,
                        &page_owner->gfp_mask, page_owner->pid,
                        page_owner->tgid, page_owner->comm,
                        page_owner->ts_nsec);

        /* Print information relevant to grouping pages by mobility */
        pageblock_mt = get_pageblock_migratetype(page);
        page_mt  = gfp_migratetype(page_owner->gfp_mask);
        ret += scnprintf(kbuf + ret, count - ret,
                        "PFN 0x%lx type %s Block %lu type %s Flags %pGp\n",
                        pfn,
                        migratetype_names[page_mt],
                        pfn >> pageblock_order,
                        migratetype_names[pageblock_mt],
                        &page->flags);

        ret += stack_depot_snprint(handle, kbuf + ret, count - ret, 0);
        if (ret >= count)
                goto err;

        if (page_owner->last_migrate_reason != -1) {
                ret += scnprintf(kbuf + ret, count - ret,
                        "Page has been migrated, last migrate reason: %s\n",
                        migrate_reason_names[page_owner->last_migrate_reason]);
        }

        ret = print_page_owner_memcg(kbuf, count, ret, page);

        ret += snprintf(kbuf + ret, count - ret, "\n");
        if (ret >= count)
                goto err;

        if (copy_to_user(buf, kbuf, ret))
                ret = -EFAULT;

        kfree(kbuf);
        return ret;

err:
        kfree(kbuf);
        return -ENOMEM;
}

void __dump_page_owner(const struct page *page)
{
        struct page_ext *page_ext = page_ext_get((void *)page);
        struct page_owner *page_owner;
        depot_stack_handle_t handle;
        gfp_t gfp_mask;
        int mt;

        if (unlikely(!page_ext)) {
                pr_alert("There is not page extension available.\n");
                return;
        }

        page_owner = get_page_owner(page_ext);
        gfp_mask = page_owner->gfp_mask;
        mt = gfp_migratetype(gfp_mask);

        if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags)) {
                pr_alert("page_owner info is not present (never set?)\n");
                page_ext_put(page_ext);
                return;
        }

        if (test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
                pr_alert("page_owner tracks the page as allocated\n");
        else
                pr_alert("page_owner tracks the page as freed\n");

        pr_alert("page last allocated via order %u, migratetype %s, gfp_mask %#x(%pGg), pid %d, tgid %d (%s), ts %llu, free_ts %llu\n",
                 page_owner->order, migratetype_names[mt], gfp_mask, &gfp_mask,
                 page_owner->pid, page_owner->tgid, page_owner->comm,
                 page_owner->ts_nsec, page_owner->free_ts_nsec);

        handle = READ_ONCE(page_owner->handle);
        if (!handle)
                pr_alert("page_owner allocation stack trace missing\n");
        else
                stack_depot_print(handle);

        handle = READ_ONCE(page_owner->free_handle);
        if (!handle) {
                pr_alert("page_owner free stack trace missing\n");
        } else {
                pr_alert("page last free pid %d tgid %d stack trace:\n",
                          page_owner->free_pid, page_owner->free_tgid);
                stack_depot_print(handle);
        }

        if (page_owner->last_migrate_reason != -1)
                pr_alert("page has been migrated, last migrate reason: %s\n",
                        migrate_reason_names[page_owner->last_migrate_reason]);
        page_ext_put(page_ext);
}

static ssize_t
read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
        unsigned long pfn;
        struct page *page;
        struct page_ext *page_ext;
        struct page_owner *page_owner;
        depot_stack_handle_t handle;

        if (!static_branch_unlikely(&page_owner_inited))
                return -EINVAL;

        page = NULL;
        if (*ppos == 0)
                pfn = min_low_pfn;
        else
                pfn = *ppos;
        /* Find a valid PFN or the start of a MAX_ORDER_NR_PAGES area */
        while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
                pfn++;

        /* Find an allocated page */
        for (; pfn < max_pfn; pfn++) {
                /*
                 * This temporary page_owner is required so
                 * that we can avoid the context switches while holding
                 * the rcu lock and copying the page owner information to
                 * user through copy_to_user() or GFP_KERNEL allocations.
                 */
                struct page_owner page_owner_tmp;

                /*
                 * If the new page is in a new MAX_ORDER_NR_PAGES area,
                 * validate the area as existing, skip it if not
                 */
                if ((pfn & (MAX_ORDER_NR_PAGES - 1)) == 0 && !pfn_valid(pfn)) {
                        pfn += MAX_ORDER_NR_PAGES - 1;
                        continue;
                }

                page = pfn_to_page(pfn);
                if (PageBuddy(page)) {
                        unsigned long freepage_order = buddy_order_unsafe(page);

                        if (freepage_order <= MAX_PAGE_ORDER)
                                pfn += (1UL << freepage_order) - 1;
                        continue;
                }

                page_ext = page_ext_get(page);
                if (unlikely(!page_ext))
                        continue;

                /*
                 * Some pages could be missed by concurrent allocation or free,
                 * because we don't hold the zone lock.
                 */
                if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
                        goto ext_put_continue;

                /*
                 * Although we do have the info about past allocation of free
                 * pages, it's not relevant for current memory usage.
                 */
                if (!test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
                        goto ext_put_continue;

                page_owner = get_page_owner(page_ext);

                /*
                 * Don't print "tail" pages of high-order allocations as that
                 * would inflate the stats.
                 */
                if (!IS_ALIGNED(pfn, 1 << page_owner->order))
                        goto ext_put_continue;

                /*
                 * Access to page_ext->handle isn't synchronous so we should
                 * be careful to access it.
                 */
                handle = READ_ONCE(page_owner->handle);
                if (!handle)
                        goto ext_put_continue;

                /* Record the next PFN to read in the file offset */
                *ppos = pfn + 1;

                page_owner_tmp = *page_owner;
                page_ext_put(page_ext);
                return print_page_owner(buf, count, pfn, page,
                                &page_owner_tmp, handle);
ext_put_continue:
                page_ext_put(page_ext);
        }

        return 0;
}

static loff_t lseek_page_owner(struct file *file, loff_t offset, int orig)
{
        switch (orig) {
        case SEEK_SET:
                file->f_pos = offset;
                break;
        case SEEK_CUR:
                file->f_pos += offset;
                break;
        default:
                return -EINVAL;
        }
        return file->f_pos;
}

static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
{
        unsigned long pfn = zone->zone_start_pfn;
        unsigned long end_pfn = zone_end_pfn(zone);
        unsigned long count = 0;

        /*
         * Walk the zone in pageblock_nr_pages steps. If a page block spans
         * a zone boundary, it will be double counted between zones. This does
         * not matter as the mixed block count will still be correct
         */
        for (; pfn < end_pfn; ) {
                unsigned long block_end_pfn;

                if (!pfn_valid(pfn)) {
                        pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES);
                        continue;
                }

                block_end_pfn = pageblock_end_pfn(pfn);
                block_end_pfn = min(block_end_pfn, end_pfn);

                for (; pfn < block_end_pfn; pfn++) {
                        struct page *page = pfn_to_page(pfn);
                        struct page_ext *page_ext;

                        if (page_zone(page) != zone)
                                continue;

                        /*
                         * To avoid having to grab zone->lock, be a little
                         * careful when reading buddy page order. The only
                         * danger is that we skip too much and potentially miss
                         * some early allocated pages, which is better than
                         * heavy lock contention.
                         */
                        if (PageBuddy(page)) {
                                unsigned long order = buddy_order_unsafe(page);

                                if (order > 0 && order <= MAX_PAGE_ORDER)
                                        pfn += (1UL << order) - 1;
                                continue;
                        }

                        if (PageReserved(page))
                                continue;

                        page_ext = page_ext_get(page);
                        if (unlikely(!page_ext))
                                continue;

                        /* Maybe overlapping zone */
                        if (test_bit(PAGE_EXT_OWNER, &page_ext->flags))
                                goto ext_put_continue;

                        /* Found early allocated page */
                        __update_page_owner_handle(page, early_handle, 0, 0,
                                                   -1, local_clock(), current->pid,
                                                   current->tgid, current->comm);
                        count++;
ext_put_continue:
                        page_ext_put(page_ext);
                }
                cond_resched();
        }

        pr_info("Node %d, zone %8s: page owner found early allocated %lu pages\n",
                pgdat->node_id, zone->name, count);
}

static void init_zones_in_node(pg_data_t *pgdat)
{
        struct zone *zone;
        struct zone *node_zones = pgdat->node_zones;

        for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
                if (!populated_zone(zone))
                        continue;

                init_pages_in_zone(pgdat, zone);
        }
}

static void init_early_allocated_pages(void)
{
        pg_data_t *pgdat;

        for_each_online_pgdat(pgdat)
                init_zones_in_node(pgdat);
}

static const struct file_operations proc_page_owner_operations = {
        .read           = read_page_owner,
        .llseek         = lseek_page_owner,
};

static void *stack_start(struct seq_file *m, loff_t *ppos)
{
        struct stack *stack;

        if (*ppos == -1UL)
                return NULL;

        if (!*ppos) {
                /*
                 * This pairs with smp_store_release() from function
                 * add_stack_record_to_list(), so we get a consistent
                 * value of stack_list.
                 */
                stack = smp_load_acquire(&stack_list);
                m->private = stack;
        } else {
                stack = m->private;
        }

        return stack;
}

static void *stack_next(struct seq_file *m, void *v, loff_t *ppos)
{
        struct stack *stack = v;

        stack = stack->next;
        *ppos = stack ? *ppos + 1 : -1UL;
        m->private = stack;

        return stack;
}

static unsigned long page_owner_pages_threshold;

static int stack_print(struct seq_file *m, void *v)
{
        int i, nr_base_pages;
        struct stack *stack = v;
        unsigned long *entries;
        unsigned long nr_entries;
        struct stack_record *stack_record = stack->stack_record;

        if (!stack->stack_record)
                return 0;

        nr_entries = stack_record->size;
        entries = stack_record->entries;
        nr_base_pages = refcount_read(&stack_record->count) - 1;

        if (nr_base_pages < 1 || nr_base_pages < page_owner_pages_threshold)
                return 0;

        for (i = 0; i < nr_entries; i++)
                seq_printf(m, " %pS\n", (void *)entries[i]);
        seq_printf(m, "nr_base_pages: %d\n\n", nr_base_pages);

        return 0;
}

static void stack_stop(struct seq_file *m, void *v)
{
}

static const struct seq_operations page_owner_stack_op = {
        .start  = stack_start,
        .next   = stack_next,
        .stop   = stack_stop,
        .show   = stack_print
};

static int page_owner_stack_open(struct inode *inode, struct file *file)
{
        return seq_open_private(file, &page_owner_stack_op, 0);
}

static const struct file_operations page_owner_stack_operations = {
        .open           = page_owner_stack_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = seq_release,
};

static int page_owner_threshold_get(void *data, u64 *val)
{
        *val = READ_ONCE(page_owner_pages_threshold);
        return 0;
}

static int page_owner_threshold_set(void *data, u64 val)
{
        WRITE_ONCE(page_owner_pages_threshold, val);
        return 0;
}

DEFINE_SIMPLE_ATTRIBUTE(proc_page_owner_threshold, &page_owner_threshold_get,
                        &page_owner_threshold_set, "%llu");


static int __init pageowner_init(void)
{
        struct dentry *dir;

        if (!static_branch_unlikely(&page_owner_inited)) {
                pr_info("page_owner is disabled\n");
                return 0;
        }

        debugfs_create_file("page_owner", 0400, NULL, NULL,
                            &proc_page_owner_operations);
        dir = debugfs_create_dir("page_owner_stacks", NULL);
        debugfs_create_file("show_stacks", 0400, dir, NULL,
                            &page_owner_stack_operations);
        debugfs_create_file("count_threshold", 0600, dir, NULL,
                            &proc_page_owner_threshold);

        return 0;
}
late_initcall(pageowner_init)