Direct Page reclaim and Background Page reclaim call path

Direct Page Reclaim call path

__get_free_pages ->
alloc_pages ->
  alloc_pages_nodemask ->
     __alloc_pages_slowpath ->
         __alloc_pages_direct_reclaim ->
            try_to_free_pages ->
                do_try_to_free_pages ->
                  shrink_slab/shrink_zones ->
                        shrink_zone ->
                            shrink_list ->
                               shrink_inactive/active_list ->
                                   shrink_page_list ->
                                        page_out ->
                                                |
                                               V
                      mapping->a_ops->writepage


Background Page Reclaim call path


wakeup_kswapd ->
kswapd ->
  balance_pgdat ->
    shrink_slab/shrink_zone ->
       shrink_list ->
          shrink_inactive/active_list ->
             shrink_page_list ->
                page_out ->
                  |
                  V
    
   mapping->a_ops->writepage



Note: Pages is moved from active list to inactive list for freeing in the end.
, shrink_active_list move pages to inactive list, when moving pages, pages are
isolated from lru list to a private list (page_list or l_hold).