/*
 *  linux/mm/swap_state.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 *
 *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
 */
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/backing-dev.h>
#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/migrate.h>

#include <asm/pgtable.h>

/*
 * swapper_space is a fiction, retained to simplify the path through
 * vmscan's shrink_page_list.
 */
static const struct address_space_operations swap_aops = {
        .writepage      = swap_writepage,
        .set_page_dirty = swap_set_page_dirty,
#ifdef CONFIG_MIGRATION
        .migratepage    = migrate_page,
#endif
};

struct address_space swapper_spaces[MAX_SWAPFILES] = {
        [0 ... MAX_SWAPFILES - 1] = {
                .page_tree      = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
                .i_mmap_writable = ATOMIC_INIT(0),
                .a_ops          = &swap_aops,
        }
};

#define INC_CACHE_INFO(x)       do { swap_cache_info.x++; } while (0)

static struct {
        unsigned long add_total;
        unsigned long del_total;
        unsigned long find_success;
        unsigned long find_total;
} swap_cache_info;

unsigned long total_swapcache_pages(void)
{
        int i;
        unsigned long ret = 0;

        for (i = 0; i < MAX_SWAPFILES; i++)
                ret += swapper_spaces[i].nrpages;
        return ret;
}

static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);

void show_swap_cache_info(void)
{
        printk("%lu pages in swap cache\n", total_swapcache_pages());
        printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
                swap_cache_info.add_total, swap_cache_info.del_total,
                swap_cache_info.find_success, swap_cache_info.find_total);
        printk("Free swap  = %ldkB\n",
                get_nr_swap_pages() << (PAGE_SHIFT - 10));
        printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
}

/*
 * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
 * but sets SwapCache flag and private instead of mapping and index.
 */
int __add_to_swap_cache(struct page *page, swp_entry_t entry)
{
        int error;
        struct address_space *address_space;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(PageSwapCache(page), page);
        VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

        page_cache_get(page);
        SetPageSwapCache(page);
        set_page_private(page, entry.val);

        address_space = swap_address_space(entry);
        spin_lock_irq(&address_space->tree_lock);
        error = radix_tree_insert(&address_space->page_tree,
                                        entry.val, page);
        if (likely(!error)) {
                address_space->nrpages++;
                __inc_zone_page_state(page, NR_FILE_PAGES);
                INC_CACHE_INFO(add_total);
        }
        spin_unlock_irq(&address_space->tree_lock);

        if (unlikely(error)) {
                /*
                 * Only the context which have set SWAP_HAS_CACHE flag
                 * would call add_to_swap_cache().
                 * So add_to_swap_cache() doesn't returns -EEXIST.
                 */
                VM_BUG_ON(error == -EEXIST);
                set_page_private(page, 0UL);
                ClearPageSwapCache(page);
                page_cache_release(page);
        }

        return error;
}


int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
{
        int error;

        error = radix_tree_maybe_preload(gfp_mask);
        if (!error) {
                error = __add_to_swap_cache(page, entry);
                radix_tree_preload_end();
        }
        return error;
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache.
 */
void __delete_from_swap_cache(struct page *page)
{
        swp_entry_t entry;
        struct address_space *address_space;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageSwapCache(page), page);
        VM_BUG_ON_PAGE(PageWriteback(page), page);

        entry.val = page_private(page);
        address_space = swap_address_space(entry);
        radix_tree_delete(&address_space->page_tree, page_private(page));
        set_page_private(page, 0);
        ClearPageSwapCache(page);
        address_space->nrpages--;
        __dec_zone_page_state(page, NR_FILE_PAGES);
        INC_CACHE_INFO(del_total);
}

/**
 * add_to_swap - allocate swap space for a page
 * @page: page we want to move to swap
 *
 * Allocate swap space for the page and add the page to the
 * swap cache.  Caller needs to hold the page lock. 
 */
int add_to_swap(struct page *page, struct list_head *list)
{
        swp_entry_t entry;
        int err;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageUptodate(page), page);

        entry = get_swap_page();
        if (!entry.val)
                return 0;

        if (unlikely(PageTransHuge(page)))
                if (unlikely(split_huge_page_to_list(page, list))) {
                        swapcache_free(entry);
                        return 0;
                }

        /*
         * Radix-tree node allocations from PF_MEMALLOC contexts could
         * completely exhaust the page allocator. __GFP_NOMEMALLOC
         * stops emergency reserves from being allocated.
         *
         * TODO: this could cause a theoretical memory reclaim
         * deadlock in the swap out path.
         */
        /*
         * Add it to the swap cache and mark it dirty
         */
        err = add_to_swap_cache(page, entry,
                        __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);

        if (!err) {     /* Success */
                SetPageDirty(page);
                return 1;
        } else {        /* -ENOMEM radix-tree allocation failure */
                /*
                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
                 * clear SWAP_HAS_CACHE flag.
                 */
                swapcache_free(entry);
                return 0;
        }
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache and locked.
 * It will never put the page into the free list,
 * the caller has a reference on the page.
 */
void delete_from_swap_cache(struct page *page)
{
        swp_entry_t entry;
        struct address_space *address_space;

        entry.val = page_private(page);

        address_space = swap_address_space(entry);
        spin_lock_irq(&address_space->tree_lock);
        __delete_from_swap_cache(page);
        spin_unlock_irq(&address_space->tree_lock);

        swapcache_free(entry);
        page_cache_release(page);
}

/* 
 * If we are the only user, then try to free up the swap cache. 
 * 
 * Its ok to check for PageSwapCache without the page lock
 * here because we are going to recheck again inside
 * try_to_free_swap() _with_ the lock.
 *                                      - Marcelo
 */
static inline void free_swap_cache(struct page *page)
{
        if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
                try_to_free_swap(page);
                unlock_page(page);
        }
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page.
 */
void free_page_and_swap_cache(struct page *page)
{
        free_swap_cache(page);
        page_cache_release(page);
}

/*
 * Passed an array of pages, drop them all from swapcache and then release
 * them.  They are removed from the LRU and freed if this is their last use.
 */
void free_pages_and_swap_cache(struct page **pages, int nr)
{
        struct page **pagep = pages;
        int i;

        lru_add_drain();
        for (i = 0; i < nr; i++)
                free_swap_cache(pagep[i]);
        release_pages(pagep, nr, false);
}

/*
 * Lookup a swap entry in the swap cache. A found page will be returned
 * unlocked and with its refcount incremented - we rely on the kernel
 * lock getting page table operations atomic even if we drop the page
 * lock before returning.
 */
struct page * lookup_swap_cache(swp_entry_t entry)
{
        struct page *page;

        page = find_get_page(swap_address_space(entry), entry.val);

        if (page) {
                INC_CACHE_INFO(find_success);
                if (TestClearPageReadahead(page))
                        atomic_inc(&swapin_readahead_hits);
        }

        INC_CACHE_INFO(find_total);
        return page;
}

struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                        struct vm_area_struct *vma, unsigned long addr,
                        bool *new_page_allocated)
{
        struct page *found_page, *new_page = NULL;
        struct address_space *swapper_space = swap_address_space(entry);
        int err;
        *new_page_allocated = false;

        do {
                /*
                 * First check the swap cache.  Since this is normally
                 * called after lookup_swap_cache() failed, re-calling
                 * that would confuse statistics.
                 */
                found_page = find_get_page(swapper_space, entry.val);
                if (found_page)
                        break;

                /*
                 * Get a new page to read into from swap.
                 */
                if (!new_page) {
                        new_page = alloc_page_vma(gfp_mask, vma, addr);
                        if (!new_page)
                                break;          /* Out of memory */
                }

                /*
                 * call radix_tree_preload() while we can wait.
                 */
                err = radix_tree_maybe_preload(gfp_mask & GFP_KERNEL);
                if (err)
                        break;

                /*
                 * Swap entry may have been freed since our caller observed it.
                 */
                err = swapcache_prepare(entry);
                if (err == -EEXIST) {
                        radix_tree_preload_end();
                        /*
                         * We might race against get_swap_page() and stumble
                         * across a SWAP_HAS_CACHE swap_map entry whose page
                         * has not been brought into the swapcache yet, while
                         * the other end is scheduled away waiting on discard
                         * I/O completion at scan_swap_map().
                         *
                         * In order to avoid turning this transitory state
                         * into a permanent loop around this -EEXIST case
                         * if !CONFIG_PREEMPT and the I/O completion happens
                         * to be waiting on the CPU waitqueue where we are now
                         * busy looping, we just conditionally invoke the
                         * scheduler here, if there are some more important
                         * tasks to run.
                         */
                        cond_resched();
                        continue;
                }
                if (err) {              /* swp entry is obsolete ? */
                        radix_tree_preload_end();
                        break;
                }

                /* May fail (-ENOMEM) if radix-tree node allocation failed. */
                __set_page_locked(new_page);
                SetPageSwapBacked(new_page);
                err = __add_to_swap_cache(new_page, entry);
                if (likely(!err)) {
                        radix_tree_preload_end();
                        /*
                         * Initiate read into locked page and return.
                         */
                        lru_cache_add_anon(new_page);
                        *new_page_allocated = true;
                        return new_page;
                }
                radix_tree_preload_end();
                ClearPageSwapBacked(new_page);
                __clear_page_locked(new_page);
                /*
                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
                 * clear SWAP_HAS_CACHE flag.
                 */
                swapcache_free(entry);
        } while (err != -ENOMEM);

        if (new_page)
                page_cache_release(new_page);
        return found_page;
}

/*
 * Locate a page of swap in physical memory, reserving swap cache space
 * and reading the disk if it is not already cached.
 * A failure return means that either the page allocation failed or that
 * the swap entry is no longer in use.
 */
struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                        struct vm_area_struct *vma, unsigned long addr)
{
        bool page_was_allocated;
        struct page *retpage = __read_swap_cache_async(entry, gfp_mask,
                        vma, addr, &page_was_allocated);

        if (page_was_allocated)
                swap_readpage(retpage);

        return retpage;
}

static unsigned long swapin_nr_pages(unsigned long offset)
{
        static unsigned long prev_offset;
        unsigned int pages, max_pages, last_ra;
        static atomic_t last_readahead_pages;

        max_pages = 1 << READ_ONCE(page_cluster);
        if (max_pages <= 1)
                return 1;

        /*
         * This heuristic has been found to work well on both sequential and
         * random loads, swapping to hard disk or to SSD: please don't ask
         * what the "+ 2" means, it just happens to work well, that's all.
         */
        pages = atomic_xchg(&swapin_readahead_hits, 0) + 2;
        if (pages == 2) {
                /*
                 * We can have no readahead hits to judge by: but must not get
                 * stuck here forever, so check for an adjacent offset instead
                 * (and don't even bother to check whether swap type is same).
                 */
                if (offset != prev_offset + 1 && offset != prev_offset - 1)
                        pages = 1;
                prev_offset = offset;
        } else {
                unsigned int roundup = 4;
                while (roundup < pages)
                        roundup <<= 1;
                pages = roundup;
        }

        if (pages > max_pages)
                pages = max_pages;

        /* Don't shrink readahead too fast */
        last_ra = atomic_read(&last_readahead_pages) / 2;
        if (pages < last_ra)
                pages = last_ra;
        atomic_set(&last_readahead_pages, pages);

        return pages;
}

/**
 * swapin_readahead - swap in pages in hope we need them soon
 * @entry: swap entry of this memory
 * @gfp_mask: memory allocation flags
 * @vma: user vma this address belongs to
 * @addr: target address for mempolicy
 *
 * Returns the struct page for entry and addr, after queueing swapin.
 *
 * Primitive swap readahead code. We simply read an aligned block of
 * (1 << page_cluster) entries in the swap area. This method is chosen
 * because it doesn't cost us any seek time.  We also make sure to queue
 * the 'original' request together with the readahead ones...
 *
 * This has been extended to use the NUMA policies from the mm triggering
 * the readahead.
 *
 * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
 */
struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
                        struct vm_area_struct *vma, unsigned long addr)
{
        struct page *page;
        unsigned long entry_offset = swp_offset(entry);
        unsigned long offset = entry_offset;
        unsigned long start_offset, end_offset;
        unsigned long mask;
        struct blk_plug plug;

        mask = swapin_nr_pages(offset) - 1;
        if (!mask)
                goto skip;

        /* Read a page_cluster sized and aligned cluster around offset. */
        start_offset = offset & ~mask;
        end_offset = offset | mask;
        if (!start_offset)      /* First page is swap header. */
                start_offset++;

        blk_start_plug(&plug);
        for (offset = start_offset; offset <= end_offset ; offset++) {
                /* Ok, do the async read-ahead now */
                page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
                                                gfp_mask, vma, addr);
                if (!page)
                        continue;
                if (offset != entry_offset)
                        SetPageReadahead(page);
                page_cache_release(page);
        }
        blk_finish_plug(&plug);

        lru_add_drain();        /* Push any new pages onto the LRU now */
skip:
        return read_swap_cache_async(entry, gfp_mask, vma, addr);
}