mm: thp: fix DEBUG_PAGEALLOC oops in copy_page_rep()

mm: thp: fix DEBUG_PAGEALLOC oops in copy_page_rep()

Trinity has for over a year been reporting a CONFIG_DEBUG_PAGEALLOC oops
in copy_page_rep() called from copy_user_huge_page() called from
do_huge_pmd_wp_page().

I believe this is a DEBUG_PAGEALLOC false positive, due to the source
page being split, and a tail page freed, while copy is in progress; and
not a problem without DEBUG_PAGEALLOC, since the pmd_same() check will
prevent a miscopy from being made visible.

Fix by adding get_user_huge_page() and put_user_huge_page(): reducing to
the usual get_page() and put_page() on head page in the usual config;
but get and put references to all of the tail pages when
DEBUG_PAGEALLOC.

[[email protected]: coding-style fixes]
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Tested-by: Sasha Levin <[email protected]>
Cc: Dave Jones <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>