mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped

Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? 
Yes.  Not by atomic64_t or cmpxchg games, those get difficult on 32-bit;
but if we slightly abuse subpages_mapcount by additionally demanding that
one bit be set there when the compound page is PMD-mapped, then a cascade
of two atomic ops is able to maintain the stats without bit_spin_lock.

This is harder to reason about than when bit_spin_locked, but I believe
safe; and no drift in stats detected when testing.  When there are racing
removes and adds, of course the sequence of operations is less well-
defined; but each operation on subpages_mapcount is atomically good.  What
might be disastrous, is if subpages_mapcount could ever fleetingly appear
negative: but the pte lock (or pmd lock) these rmap functions are called
under, ensures that a last remove cannot race ahead of a first add.

Continue to make an exception for hugetlb (PageHuge) pages, though that
exception can be easily removed by a further commit if necessary: leave
subpages_mapcount 0, don't bother with COMPOUND_MAPPED in its case, just
carry on checking compound_mapcount too in folio_mapped(), page_mapped().

Evidence is that this way goes slightly faster than the previous
implementation in all cases (pmds after ptes now taking around 103ms); and
relieves us of worrying about contention on the bit_spin_lock.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: James Houghton <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Zach O'Keefe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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