// SPDX-License-Identifier: GPL-2.0
/*
 * Manage cache of swap slots to be used for and returned from
 * swap.
 *
 * Copyright(c) 2016 Intel Corporation.
 *
 * Author: Tim Chen <tim.c.chen@linux.intel.com>
 *
 * We allocate the swap slots from the global pool and put
 * it into local per cpu caches.  This has the advantage
 * of no needing to acquire the swap_info lock every time
 * we need a new slot.
 *
 * There is also opportunity to simply return the slot
 * to local caches without needing to acquire swap_info
 * lock.  We do not reuse the returned slots directly but
 * move them back to the global pool in a batch.  This
 * allows the slots to coalesce and reduce fragmentation.
 *
 * The swap entry allocated is marked with SWAP_HAS_CACHE
 * flag in map_count that prevents it from being allocated
 * again from the global pool.
 *
 * The swap slots cache is protected by a mutex instead of
 * a spin lock as when we search for slots with scan_swap_map,
 * we can possibly sleep.
 */

/*
 * Official Linux v6.0: mm/swap_slots.c
 * https://github.com/torvalds/linux/blob/v6.0/mm/swap_slots.c#L302
 * Exact definition/declaration; not a reconstruction of a book listing.
 * See GPL-2.0.txt. Not a standalone translation unit.
 */

swp_entry_t folio_alloc_swap(struct folio *folio)
{
	swp_entry_t entry;
	struct swap_slots_cache *cache;

	entry.val = 0;

	if (folio_test_large(folio)) {
		if (IS_ENABLED(CONFIG_THP_SWAP) && arch_thp_swp_supported())
			get_swap_pages(1, &entry, folio_nr_pages(folio));
		goto out;
	}

	/*
	 * Preemption is allowed here, because we may sleep
	 * in refill_swap_slots_cache().  But it is safe, because
	 * accesses to the per-CPU data structure are protected by the
	 * mutex cache->alloc_lock.
	 *
	 * The alloc path here does not touch cache->slots_ret
	 * so cache->free_lock is not taken.
	 */
	cache = raw_cpu_ptr(&swp_slots);

	if (likely(check_cache_active() && cache->slots)) {
		mutex_lock(&cache->alloc_lock);
		if (cache->slots) {
repeat:
			if (cache->nr) {
				entry = cache->slots[cache->cur];
				cache->slots[cache->cur++].val = 0;
				cache->nr--;
			} else if (refill_swap_slots_cache(cache)) {
				goto repeat;
			}
		}
		mutex_unlock(&cache->alloc_lock);
		if (entry.val)
			goto out;
	}

	get_swap_pages(1, &entry, 1);
out:
	if (mem_cgroup_try_charge_swap(folio, entry)) {
		put_swap_page(&folio->page, entry);
		entry.val = 0;
	}
	return entry;
}