From fs/proc/task_mmu.c (Linux kernel v6.9.6)

2557 static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty,2557                         unsigned long nr_pages)
2558 {
2559         int count = page_mapcount(page);
2560
2561         md->pages += nr_pages;
2562         if (pte_dirty || PageDirty(page))
2563                 md->dirty += nr_pages;
2564
2565         if (PageSwapCache(page))
2566                 md->swapcache += nr_pages;
2567
2568         if (PageActive(page) || PageUnevictable(page))
2569                 md->active += nr_pages;
2570
2571         if (PageWriteback(page))
2572                 md->writeback += nr_pages;
2573
2574         if (PageAnon(page))
2575                 md->anon += nr_pages;
2576
2577         if (count > md->mapcount_max)
2578                 md->mapcount_max = count;
2579
2580         md->node[page_to_nid(page)] += nr_pages;
2581 }

All these Page{Active,Writeback,...}(page) when I jump to in VS Code:

PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
    TESTCLEARFLAG(Active, active, PF_HEAD)

And if I grep -R PageActive it is defined nowhere.

How does this work? Is this a GCC specific thing?