// SPDX-License-Identifier: GPL-2.0-only
/*
 * Complete definition from Linux v6.18 mm/filemap.c.
 * Source: https://github.com/torvalds/linux/blob/v6.18/mm/filemap.c
 * Copyright (C) 1994-1999  Linus Torvalds
 * Kernel excerpt; not a standalone user-space program.
 * The book's exact source tag is not asserted.
 */

static void filemap_unaccount_folio(struct address_space *mapping,
		struct folio *folio)
{
	long nr;

	VM_BUG_ON_FOLIO(folio_mapped(folio), folio);
	if (!IS_ENABLED(CONFIG_DEBUG_VM) && unlikely(folio_mapped(folio))) {
		pr_alert("BUG: Bad page cache in process %s  pfn:%05lx\n",
			 current->comm, folio_pfn(folio));
		dump_page(&folio->page, "still mapped when deleted");
		dump_stack();
		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);

		if (mapping_exiting(mapping) && !folio_test_large(folio)) {
			int mapcount = folio_mapcount(folio);

			if (folio_ref_count(folio) >= mapcount + 2) {
				/*
				 * All vmas have already been torn down, so it's
				 * a good bet that actually the page is unmapped
				 * and we'd rather not leak it: if we're wrong,
				 * another bad page check should catch it later.
				 */
				atomic_set(&folio->_mapcount, -1);
				folio_ref_sub(folio, mapcount);
			}
		}
	}

	/* hugetlb folios do not participate in page cache accounting. */
	if (folio_test_hugetlb(folio))
		return;

	nr = folio_nr_pages(folio);

	__lruvec_stat_mod_folio(folio, NR_FILE_PAGES, -nr);
	if (folio_test_swapbacked(folio)) {
		__lruvec_stat_mod_folio(folio, NR_SHMEM, -nr);
		if (folio_test_pmd_mappable(folio))
			__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr);
	} else if (folio_test_pmd_mappable(folio)) {
		__lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
		filemap_nr_thps_dec(mapping);
	}
	if (test_bit(AS_KERNEL_FILE, &folio->mapping->flags))
		mod_node_page_state(folio_pgdat(folio),
				    NR_KERNEL_FILE_PAGES, -nr);

	/*
	 * At this point folio must be either written or cleaned by
	 * truncate.  Dirty folio here signals a bug and loss of
	 * unwritten data - on ordinary filesystems.
	 *
	 * But it's harmless on in-memory filesystems like tmpfs; and can
	 * occur when a driver which did get_user_pages() sets page dirty
	 * before putting it, while the inode is being finally evicted.
	 *
	 * Below fixes dirty accounting after removing the folio entirely
	 * but leaves the dirty flag set: it has no effect for truncated
	 * folio and anyway will be cleared before returning folio to
	 * buddy allocator.
	 */
	if (WARN_ON_ONCE(folio_test_dirty(folio) &&
			 mapping_can_writeback(mapping)))
		folio_account_cleaned(folio, inode_to_wb(mapping->host));
}