mm/khugepaged: record SCAN_PMD_MAPPED when scan_pmd() finds hugepage

When scanning an anon pmd to see if it's eligible for collapse, return
SCAN_PMD_MAPPED if the pmd already maps a hugepage.  Note that
SCAN_PMD_MAPPED is different from SCAN_PAGE_COMPOUND used in the
file-collapse path, since the latter might identify pte-mapped compound
pages.  This is required by MADV_COLLAPSE which necessarily needs to know
what hugepage-aligned/sized regions are already pmd-mapped.

In order to determine if a pmd already maps a hugepage, refactor
mm_find_pmd():

Return mm_find_pmd() to it's pre-commit f72e7dcdd252 ("mm: let mm_find_pmd
fix buggy race with THP fault") behavior.  ksm was the only caller that
explicitly wanted a pte-mapping pmd, so open code the pte-mapping logic
there (pmd_present() and pmd_trans_huge() checks).

Undo revert change in commit f72e7dcdd252 ("mm: let mm_find_pmd fix buggy
race with THP fault") that open-coded split_huge_pmd_address() pmd lookup
and use mm_find_pmd() instead.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zach O'Keefe <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Chris Kennelly <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Pavel Begunkov <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Rongwei Wang <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: "Souptick Joarder (HPE)" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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