// SPDX-License-Identifier: GPL-2.0
// Copyright(c) 2018 Intel Corporation. All rights reserved.

#include <linux/mm.h>
#include <linux/init.h>
#include <linux/mmzone.h>
#include <linux/random.h>
#include <linux/moduleparam.h>
#include "internal.h"
#include "shuffle.h"

DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);

static bool shuffle_param;
static int shuffle_show(char *buffer, const struct kernel_param *kp)
{
        return sprintf(buffer, "%c\n", shuffle_param ? 'Y' : 'N');
}

static __meminit int shuffle_store(const char *val,
                const struct kernel_param *kp)
{
        int rc = param_set_bool(val, kp);

        if (rc < 0)
                return rc;
        if (shuffle_param)
                static_branch_enable(&page_alloc_shuffle_key);
        return 0;
}
module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);

/*
 * For two pages to be swapped in the shuffle, they must be free (on a
 * 'free_area' lru), have the same order, and have the same migratetype.
 */
static struct page * __meminit shuffle_valid_page(struct zone *zone,
                                                  unsigned long pfn, int order)
{
        struct page *page = pfn_to_online_page(pfn);

        /*
         * Given we're dealing with randomly selected pfns in a zone we
         * need to ask questions like...
         */

        /* ... is the page managed by the buddy? */
        if (!page)
                return NULL;

        /* ... is the page assigned to the same zone? */
        if (page_zone(page) != zone)
                return NULL;

        /* ...is the page free and currently on a free_area list? */
        if (!PageBuddy(page))
                return NULL;

        /*
         * ...is the page on the same list as the page we will
         * shuffle it with?
         */
        if (buddy_order(page) != order)
                return NULL;

        return page;
}

/*
 * Fisher-Yates shuffle the freelist which prescribes iterating through an
 * array, pfns in this case, and randomly swapping each entry with another in
 * the span, end_pfn - start_pfn.
 *
 * To keep the implementation simple it does not attempt to correct for sources
 * of bias in the distribution, like modulo bias or pseudo-random number
 * generator bias. I.e. the expectation is that this shuffling raises the bar
 * for attacks that exploit the predictability of page allocations, but need not
 * be a perfect shuffle.
 */
#define SHUFFLE_RETRY 10
void __meminit __shuffle_zone(struct zone *z)
{
        unsigned long i, flags;
        unsigned long start_pfn = z->zone_start_pfn;
        unsigned long end_pfn = zone_end_pfn(z);
        const int order = SHUFFLE_ORDER;
        const int order_pages = 1 << order;

        spin_lock_irqsave(&z->lock, flags);
        start_pfn = ALIGN(start_pfn, order_pages);
        for (i = start_pfn; i < end_pfn; i += order_pages) {
                unsigned long j;
                int migratetype, retry;
                struct page *page_i, *page_j;

                /*
                 * We expect page_i, in the sub-range of a zone being added
                 * (@start_pfn to @end_pfn), to more likely be valid compared to
                 * page_j randomly selected in the span @zone_start_pfn to
                 * @spanned_pages.
                 */
                page_i = shuffle_valid_page(z, i, order);
                if (!page_i)
                        continue;

                for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
                        /*
                         * Pick a random order aligned page in the zone span as
                         * a swap target. If the selected pfn is a hole, retry
                         * up to SHUFFLE_RETRY attempts find a random valid pfn
                         * in the zone.
                         */
                        j = z->zone_start_pfn +
                                ALIGN_DOWN(get_random_long() % z->spanned_pages,
                                                order_pages);
                        page_j = shuffle_valid_page(z, j, order);
                        if (page_j && page_j != page_i)
                                break;
                }
                if (retry >= SHUFFLE_RETRY) {
                        pr_debug("%s: failed to swap %#lx\n", __func__, i);
                        continue;
                }

                /*
                 * Each migratetype corresponds to its own list, make sure the
                 * types match otherwise we're moving pages to lists where they
                 * do not belong.
                 */
                migratetype = get_pageblock_migratetype(page_i);
                if (get_pageblock_migratetype(page_j) != migratetype) {
                        pr_debug("%s: migratetype mismatch %#lx\n", __func__, i);
                        continue;
                }

                list_swap(&page_i->lru, &page_j->lru);

                pr_debug("%s: swap: %#lx -> %#lx\n", __func__, i, j);

                /* take it easy on the zone lock */
                if ((i % (100 * order_pages)) == 0) {
                        spin_unlock_irqrestore(&z->lock, flags);
                        cond_resched();
                        spin_lock_irqsave(&z->lock, flags);
                }
        }
        spin_unlock_irqrestore(&z->lock, flags);
}

/*
 * __shuffle_free_memory - reduce the predictability of the page allocator
 * @pgdat: node page data
 */
void __meminit __shuffle_free_memory(pg_data_t *pgdat)
{
        struct zone *z;

        for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
                shuffle_zone(z);
}

bool shuffle_pick_tail(void)
{
        static u64 rand;
        static u8 rand_bits;
        bool ret;

        /*
         * The lack of locking is deliberate. If 2 threads race to
         * update the rand state it just adds to the entropy.
         */
        if (rand_bits == 0) {
                rand_bits = 64;
                rand = get_random_u64();
        }

        ret = rand & 1;

        rand_bits--;
        rand >>= 1;

        return ret;
}