// SPDX-License-Identifier: GPL-2.0
/*
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *
 *  Swap reorganised 29.12.95, Stephen Tweedie.
 *  kswapd added: 7.1.96  sct
 *  Removed kswapd_ctl limits, and swap out as many pages as needed
 *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
 *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
 *  Multiqueue VM started 5.8.00, Rik van Riel.
 */

/* Extracted unchanged from Linux v6.18, mm/vmscan.c:6487-6521.
 * Complete function definition; surrounding kernel context is required.
 * Source: https://github.com/torvalds/linux/blob/v6.18/mm/vmscan.c#L6487
 * License text: GPL-2.0.txt (in this directory).
 */
static bool allow_direct_reclaim(pg_data_t *pgdat)
{
	struct zone *zone;
	unsigned long pfmemalloc_reserve = 0;
	unsigned long free_pages = 0;
	int i;
	bool wmark_ok;

	if (atomic_read(&pgdat->kswapd_failures) >= MAX_RECLAIM_RETRIES)
		return true;

	for_each_managed_zone_pgdat(zone, pgdat, i, ZONE_NORMAL) {
		if (!zone_reclaimable_pages(zone) && zone_page_state_snapshot(zone, NR_FREE_PAGES))
			continue;

		pfmemalloc_reserve += min_wmark_pages(zone);
		free_pages += zone_page_state_snapshot(zone, NR_FREE_PAGES);
	}

	/* If there are no reserves (unexpected config) then do not throttle */
	if (!pfmemalloc_reserve)
		return true;

	wmark_ok = free_pages > pfmemalloc_reserve / 2;

	/* kswapd must be awake if processes are being throttled */
	if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
		if (READ_ONCE(pgdat->kswapd_highest_zoneidx) > ZONE_NORMAL)
			WRITE_ONCE(pgdat->kswapd_highest_zoneidx, ZONE_NORMAL);

		wake_up_interruptible(&pgdat->kswapd_wait);
	}

	return wmark_ok;
}