mm: page-writeback: simplify memcg handling in test_clear_page_writeback()

Page writeback doesn't hold a page reference, which allows truncate to
free a page the second PageWriteback is cleared.  This used to require
special attention in test_clear_page_writeback(), where we had to be
careful not to rely on the unstable page->memcg binding and look up all
the necessary information before clearing the writeback flag.

Since commit 073861ed ("mm: fix VM_BUG_ON(PageTail) and
BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
explicit reference on the page, and this dance is no longer needed.

Use unlock_page_memcg() and dec_lruvec_page_state() directly.

This removes the last user of the lock_page_memcg() return value, change
it to void.  Touch up the comments in there as well.  This also removes
the last extern user of __unlock_page_memcg(), make it static.  Further,
it removes the last user of dec_lruvec_state(), delete it, along with a
few other unused helpers.

Link: https://lkml.kernel.org/r/YCQbYAWg4nvBFL6h@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>