- ->_refcount in tail pages is always zero: get_page_unless_zero() never
succeeds on tail pages.
- - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
- on relevant sub-page of the compound page.
-
- - map/unmap of the whole compound page is accounted for in compound_mapcount
- (stored in first tail page). For file huge pages, we also increment
- ->_mapcount of all sub-pages in order to have race-free detection of
- last unmap of subpages.
-
-PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.
-
-For anonymous pages, PageDoubleMap() also indicates ->_mapcount in all
-subpages is offset up by one. This additional reference is required to
-get race-free detection of unmap of subpages when we have them mapped with
-both PMDs and PTEs.
-
-This optimization is required to lower the overhead of per-subpage mapcount
-tracking. The alternative is to alter ->_mapcount in all subpages on each
-map/unmap of the whole compound page.
-
-For anonymous pages, we set PG_double_map when a PMD of the page is split
-for the first time, but still have a PMD mapping. The additional references
-go away with the last compound_mapcount.
-
-File pages get PG_double_map set on the first map of the page with PTE and
-goes away when the page gets evicted from the page cache.
+ - map/unmap of PMD entry for the whole compound page increment/decrement
+ ->compound_mapcount, stored in the first tail page of the compound page.
+
+ - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount
+ on relevant sub-page of the compound page, and also increment/decrement
+ ->subpages_mapcount, stored in first tail page of the compound page.
+ In order to have race-free accounting of sub-pages mapped, changes to
+ sub-page ->_mapcount, ->subpages_mapcount and ->compound_mapcount are
+ are all locked by bit_spin_lock of PG_locked in the first tail ->flags.
split_huge_page internally has to distribute the refcounts in the head
page to the tail pages before clearing all PG_head/tail bits from the page