Fill in missing pieces: reimplementation of munlock_vma_pages_range(),
required to lower the mlock_counts when munlocking without munmapping;
and its complement, implementation of mlock_vma_pages_range(), required
to raise the mlock_counts on pages already there when a range is mlocked.

Combine them into just the one function mlock_vma_pages_range(), using
walk_page_range() to run mlock_pte_range().  This approach fixes the
"Very slow unlockall()" of unpopulated PROT_NONE areas, reported in
https://lore.kernel.org/linux-mm/70885d37-62b7-748b-29df-9e94f3291736@gmail.com/

Munlock clears VM_LOCKED at the start, under exclusive mmap_lock; but if
a racing truncate or holepunch (depending on i_mmap_rwsem) gets to the
pte first, it will not try to munlock the page: leaving release_pages()
to correct it when the last reference to the page is gone - that's okay,
a page is not evictable anyway while it is held by an extra reference.

Mlock sets VM_LOCKED at the start, under exclusive mmap_lock; but if
a racing remove_migration_pte() or try_to_unmap_one() (depending on
i_mmap_rwsem) gets to the pte first, it will try to mlock the page,
then mlock_pte_range() mlock it a second time.  This is harder to
reproduce, but a more serious race because it could leave the page
unevictable indefinitely though the area is munlocked afterwards.
Guard against it by setting the (inappropriate) VM_IO flag,
and modifying mlock_vma_page() to decline such vmas.
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>