[PATCHv4 06/24] mm: store mapcount for compound page separately

We're going to allow mapping of individual 4k pages of THP compound and we need a cheap way to find out how many time the compound page is mapped with PMD -- compound_mapcount() does this. We use the same approach as with compound page destructor and compound order: use space in first tail page,

[PATCHv4 06/24] mm: store mapcount for compound page separately

We're going to allow mapping of individual 4k pages of THP compound and we need a cheap way to find out how many time the compound page is mapped with PMD -- compound_mapcount() does this. We use the same approach as with compound page destructor and compound order: use space in first tail page,