/*
 * Frontswap frontend
 *
 * This code provides the generic "frontend" layer to call a matching
 * "backend" driver implementation of frontswap.  See
 * Documentation/vm/frontswap.rst for more information.
 *
 * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
 * Author: Dan Magenheimer
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 */

#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/security.h>
#include <linux/module.h>
#include <linux/debugfs.h>
#include <linux/frontswap.h>
#include <linux/swapfile.h>

DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);

/*
 * frontswap_ops are added by frontswap_register_ops, and provide the
 * frontswap "backend" implementation functions.  Multiple implementations
 * may be registered, but implementations can never deregister.  This
 * is a simple singly-linked list of all registered implementations.
 */
static struct frontswap_ops *frontswap_ops __read_mostly;

#define for_each_frontswap_ops(ops)             \
        for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)

/*
 * If enabled, frontswap_store will return failure even on success.  As
 * a result, the swap subsystem will always write the page to swap, in
 * effect converting frontswap into a writethrough cache.  In this mode,
 * there is no direct reduction in swap writes, but a frontswap backend
 * can unilaterally "reclaim" any pages in use with no data loss, thus
 * providing increases control over maximum memory usage due to frontswap.
 */
static bool frontswap_writethrough_enabled __read_mostly;

/*
 * If enabled, the underlying tmem implementation is capable of doing
 * exclusive gets, so frontswap_load, on a successful tmem_get must
 * mark the page as no longer in frontswap AND mark it dirty.
 */
static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;

#ifdef CONFIG_DEBUG_FS
/*
 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
 * properly configured).  These are for information only so are not protected
 * against increment races.
 */
static u64 frontswap_loads;
static u64 frontswap_succ_stores;
static u64 frontswap_failed_stores;
static u64 frontswap_invalidates;

static inline void inc_frontswap_loads(void) {
        frontswap_loads++;
}
static inline void inc_frontswap_succ_stores(void) {
        frontswap_succ_stores++;
}
static inline void inc_frontswap_failed_stores(void) {
        frontswap_failed_stores++;
}
static inline void inc_frontswap_invalidates(void) {
        frontswap_invalidates++;
}
#else
static inline void inc_frontswap_loads(void) { }
static inline void inc_frontswap_succ_stores(void) { }
static inline void inc_frontswap_failed_stores(void) { }
static inline void inc_frontswap_invalidates(void) { }
#endif

/*
 * Due to the asynchronous nature of the backends loading potentially
 * _after_ the swap system has been activated, we have chokepoints
 * on all frontswap functions to not call the backend until the backend
 * has registered.
 *
 * This would not guards us against the user deciding to call swapoff right as
 * we are calling the backend to initialize (so swapon is in action).
 * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
 * OK. The other scenario where calls to frontswap_store (called via
 * swap_writepage) is racing with frontswap_invalidate_area (called via
 * swapoff) is again guarded by the swap subsystem.
 *
 * While no backend is registered all calls to frontswap_[store|load|
 * invalidate_area|invalidate_page] are ignored or fail.
 *
 * The time between the backend being registered and the swap file system
 * calling the backend (via the frontswap_* functions) is indeterminate as
 * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
 * That is OK as we are comfortable missing some of these calls to the newly
 * registered backend.
 *
 * Obviously the opposite (unloading the backend) must be done after all
 * the frontswap_[store|load|invalidate_area|invalidate_page] start
 * ignoring or failing the requests.  However, there is currently no way
 * to unload a backend once it is registered.
 */

/*
 * Register operations for frontswap
 */
void frontswap_register_ops(struct frontswap_ops *ops)
{
        DECLARE_BITMAP(a, MAX_SWAPFILES);
        DECLARE_BITMAP(b, MAX_SWAPFILES);
        struct swap_info_struct *si;
        unsigned int i;

        bitmap_zero(a, MAX_SWAPFILES);
        bitmap_zero(b, MAX_SWAPFILES);

        spin_lock(&swap_lock);
        plist_for_each_entry(si, &swap_active_head, list) {
                if (!WARN_ON(!si->frontswap_map))
                        set_bit(si->type, a);
        }
        spin_unlock(&swap_lock);

        /* the new ops needs to know the currently active swap devices */
        for_each_set_bit(i, a, MAX_SWAPFILES)
                ops->init(i);

        /*
         * Setting frontswap_ops must happen after the ops->init() calls
         * above; cmpxchg implies smp_mb() which will ensure the init is
         * complete at this point.
         */
        do {
                ops->next = frontswap_ops;
        } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);

        static_branch_inc(&frontswap_enabled_key);

        spin_lock(&swap_lock);
        plist_for_each_entry(si, &swap_active_head, list) {
                if (si->frontswap_map)
                        set_bit(si->type, b);
        }
        spin_unlock(&swap_lock);

        /*
         * On the very unlikely chance that a swap device was added or
         * removed between setting the "a" list bits and the ops init
         * calls, we re-check and do init or invalidate for any changed
         * bits.
         */
        if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
                for (i = 0; i < MAX_SWAPFILES; i++) {
                        if (!test_bit(i, a) && test_bit(i, b))
                                ops->init(i);
                        else if (test_bit(i, a) && !test_bit(i, b))
                                ops->invalidate_area(i);
                }
        }
}
EXPORT_SYMBOL(frontswap_register_ops);

/*
 * Enable/disable frontswap writethrough (see above).
 */
void frontswap_writethrough(bool enable)
{
        frontswap_writethrough_enabled = enable;
}
EXPORT_SYMBOL(frontswap_writethrough);

/*
 * Enable/disable frontswap exclusive gets (see above).
 */
void frontswap_tmem_exclusive_gets(bool enable)
{
        frontswap_tmem_exclusive_gets_enabled = enable;
}
EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);

/*
 * Called when a swap device is swapon'd.
 */
void __frontswap_init(unsigned type, unsigned long *map)
{
        struct swap_info_struct *sis = swap_info[type];
        struct frontswap_ops *ops;

        VM_BUG_ON(sis == NULL);

        /*
         * p->frontswap is a bitmap that we MUST have to figure out which page
         * has gone in frontswap. Without it there is no point of continuing.
         */
        if (WARN_ON(!map))
                return;
        /*
         * Irregardless of whether the frontswap backend has been loaded
         * before this function or it will be later, we _MUST_ have the
         * p->frontswap set to something valid to work properly.
         */
        frontswap_map_set(sis, map);

        for_each_frontswap_ops(ops)
                ops->init(type);
}
EXPORT_SYMBOL(__frontswap_init);

bool __frontswap_test(struct swap_info_struct *sis,
                                pgoff_t offset)
{
        if (sis->frontswap_map)
                return test_bit(offset, sis->frontswap_map);
        return false;
}
EXPORT_SYMBOL(__frontswap_test);

static inline void __frontswap_set(struct swap_info_struct *sis,
                                   pgoff_t offset)
{
        set_bit(offset, sis->frontswap_map);
        atomic_inc(&sis->frontswap_pages);
}

static inline void __frontswap_clear(struct swap_info_struct *sis,
                                     pgoff_t offset)
{
        clear_bit(offset, sis->frontswap_map);
        atomic_dec(&sis->frontswap_pages);
}

/*
 * "Store" data from a page to frontswap and associate it with the page's
 * swaptype and offset.  Page must be locked and in the swap cache.
 * If frontswap already contains a page with matching swaptype and
 * offset, the frontswap implementation may either overwrite the data and
 * return success or invalidate the page from frontswap and return failure.
 */
int __frontswap_store(struct page *page)
{
        int ret = -1;
        swp_entry_t entry = { .val = page_private(page), };
        int type = swp_type(entry);
        struct swap_info_struct *sis = swap_info[type];
        pgoff_t offset = swp_offset(entry);
        struct frontswap_ops *ops;

        VM_BUG_ON(!frontswap_ops);
        VM_BUG_ON(!PageLocked(page));
        VM_BUG_ON(sis == NULL);

        /*
         * If a dup, we must remove the old page first; we can't leave the
         * old page no matter if the store of the new page succeeds or fails,
         * and we can't rely on the new page replacing the old page as we may
         * not store to the same implementation that contains the old page.
         */
        if (__frontswap_test(sis, offset)) {
                __frontswap_clear(sis, offset);
                for_each_frontswap_ops(ops)
                        ops->invalidate_page(type, offset);
        }

        /* Try to store in each implementation, until one succeeds. */
        for_each_frontswap_ops(ops) {
                ret = ops->store(type, offset, page);
                if (!ret) /* successful store */
                        break;
        }
        if (ret == 0) {
                __frontswap_set(sis, offset);
                inc_frontswap_succ_stores();
        } else {
                inc_frontswap_failed_stores();
        }
        if (frontswap_writethrough_enabled)
                /* report failure so swap also writes to swap device */
                ret = -1;
        return ret;
}
EXPORT_SYMBOL(__frontswap_store);

/*
 * "Get" data from frontswap associated with swaptype and offset that were
 * specified when the data was put to frontswap and use it to fill the
 * specified page with data. Page must be locked and in the swap cache.
 */
int __frontswap_load(struct page *page)
{
        int ret = -1;
        swp_entry_t entry = { .val = page_private(page), };
        int type = swp_type(entry);
        struct swap_info_struct *sis = swap_info[type];
        pgoff_t offset = swp_offset(entry);
        struct frontswap_ops *ops;

        VM_BUG_ON(!frontswap_ops);
        VM_BUG_ON(!PageLocked(page));
        VM_BUG_ON(sis == NULL);

        if (!__frontswap_test(sis, offset))
                return -1;

        /* Try loading from each implementation, until one succeeds. */
        for_each_frontswap_ops(ops) {
                ret = ops->load(type, offset, page);
                if (!ret) /* successful load */
                        break;
        }
        if (ret == 0) {
                inc_frontswap_loads();
                if (frontswap_tmem_exclusive_gets_enabled) {
                        SetPageDirty(page);
                        __frontswap_clear(sis, offset);
                }
        }
        return ret;
}
EXPORT_SYMBOL(__frontswap_load);

/*
 * Invalidate any data from frontswap associated with the specified swaptype
 * and offset so that a subsequent "get" will fail.
 */
void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
        struct swap_info_struct *sis = swap_info[type];
        struct frontswap_ops *ops;

        VM_BUG_ON(!frontswap_ops);
        VM_BUG_ON(sis == NULL);

        if (!__frontswap_test(sis, offset))
                return;

        for_each_frontswap_ops(ops)
                ops->invalidate_page(type, offset);
        __frontswap_clear(sis, offset);
        inc_frontswap_invalidates();
}
EXPORT_SYMBOL(__frontswap_invalidate_page);

/*
 * Invalidate all data from frontswap associated with all offsets for the
 * specified swaptype.
 */
void __frontswap_invalidate_area(unsigned type)
{
        struct swap_info_struct *sis = swap_info[type];
        struct frontswap_ops *ops;

        VM_BUG_ON(!frontswap_ops);
        VM_BUG_ON(sis == NULL);

        if (sis->frontswap_map == NULL)
                return;

        for_each_frontswap_ops(ops)
                ops->invalidate_area(type);
        atomic_set(&sis->frontswap_pages, 0);
        bitmap_zero(sis->frontswap_map, sis->max);
}
EXPORT_SYMBOL(__frontswap_invalidate_area);

static unsigned long __frontswap_curr_pages(void)
{
        unsigned long totalpages = 0;
        struct swap_info_struct *si = NULL;

        assert_spin_locked(&swap_lock);
        plist_for_each_entry(si, &swap_active_head, list)
                totalpages += atomic_read(&si->frontswap_pages);
        return totalpages;
}

static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
                                        int *swapid)
{
        int ret = -EINVAL;
        struct swap_info_struct *si = NULL;
        int si_frontswap_pages;
        unsigned long total_pages_to_unuse = total;
        unsigned long pages = 0, pages_to_unuse = 0;

        assert_spin_locked(&swap_lock);
        plist_for_each_entry(si, &swap_active_head, list) {
                si_frontswap_pages = atomic_read(&si->frontswap_pages);
                if (total_pages_to_unuse < si_frontswap_pages) {
                        pages = pages_to_unuse = total_pages_to_unuse;
                } else {
                        pages = si_frontswap_pages;
                        pages_to_unuse = 0; /* unuse all */
                }
                /* ensure there is enough RAM to fetch pages from frontswap */
                if (security_vm_enough_memory_mm(current->mm, pages)) {
                        ret = -ENOMEM;
                        continue;
                }
                vm_unacct_memory(pages);
                *unused = pages_to_unuse;
                *swapid = si->type;
                ret = 0;
                break;
        }

        return ret;
}

/*
 * Used to check if it's necessory and feasible to unuse pages.
 * Return 1 when nothing to do, 0 when need to shink pages,
 * error code when there is an error.
 */
static int __frontswap_shrink(unsigned long target_pages,
                                unsigned long *pages_to_unuse,
                                int *type)
{
        unsigned long total_pages = 0, total_pages_to_unuse;

        assert_spin_locked(&swap_lock);

        total_pages = __frontswap_curr_pages();
        if (total_pages <= target_pages) {
                /* Nothing to do */
                *pages_to_unuse = 0;
                return 1;
        }
        total_pages_to_unuse = total_pages - target_pages;
        return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
}

/*
 * Frontswap, like a true swap device, may unnecessarily retain pages
 * under certain circumstances; "shrink" frontswap is essentially a
 * "partial swapoff" and works by calling try_to_unuse to attempt to
 * unuse enough frontswap pages to attempt to -- subject to memory
 * constraints -- reduce the number of pages in frontswap to the
 * number given in the parameter target_pages.
 */
void frontswap_shrink(unsigned long target_pages)
{
        unsigned long pages_to_unuse = 0;
        int uninitialized_var(type), ret;

        /*
         * we don't want to hold swap_lock while doing a very
         * lengthy try_to_unuse, but swap_list may change
         * so restart scan from swap_active_head each time
         */
        spin_lock(&swap_lock);
        ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
        spin_unlock(&swap_lock);
        if (ret == 0)
                try_to_unuse(type, true, pages_to_unuse);
        return;
}
EXPORT_SYMBOL(frontswap_shrink);

/*
 * Count and return the number of frontswap pages across all
 * swap devices.  This is exported so that backend drivers can
 * determine current usage without reading debugfs.
 */
unsigned long frontswap_curr_pages(void)
{
        unsigned long totalpages = 0;

        spin_lock(&swap_lock);
        totalpages = __frontswap_curr_pages();
        spin_unlock(&swap_lock);

        return totalpages;
}
EXPORT_SYMBOL(frontswap_curr_pages);

static int __init init_frontswap(void)
{
#ifdef CONFIG_DEBUG_FS
        struct dentry *root = debugfs_create_dir("frontswap", NULL);
        if (root == NULL)
                return -ENXIO;
        debugfs_create_u64("loads", 0444, root, &frontswap_loads);
        debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
        debugfs_create_u64("failed_stores", 0444, root,
                           &frontswap_failed_stores);
        debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
#endif
        return 0;
}

module_init(init_frontswap);