// SPDX-License-Identifier: GPL-2.0
/*
 *  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
 */

/*
 * Official Linux v6.0: mm/swap_state.c
 * https://github.com/torvalds/linux/blob/v6.0/mm/swap_state.c#L528
 * Exact definition/declaration; not a reconstruction of a book listing.
 * See GPL-2.0.txt. Not a standalone translation unit.
 */

static unsigned int __swapin_nr_pages(unsigned long prev_offset,
				      unsigned long offset,
				      int hits,
				      int max_pages,
				      int prev_win)
{
	unsigned int pages, last_ra;

	/*
	 * 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 = hits + 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;
	} 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 = prev_win / 2;
	if (pages < last_ra)
		pages = last_ra;

	return pages;
}