mm/swap.c: introduce put_[un]refcounted_compound_page helpers for splitting put_compo...

mm/swap.c: introduce put_[un]refcounted_compound_page helpers for splitting put_compound_page()

Currently, put_compound_page() carefully handles tricky cases to avoid
racing with compound page releasing or splitting, which makes it quite
lenthy (about 200+ lines) and needs deep tab indention, which makes it
quite hard to follow and maintain.

This patch and the next patch refactor this function.

Based on the code skeleton of put_compound_page:

put_compound_pge:
        if !PageTail(page)
        	put head page fastpath;
		return;

        /* else PageTail */
        page_head = compound_head(page)
        if !__compound_tail_refcounted(page_head)
		put head page optimal path; <---(1)
		return;
        else
		put head page slowpath; <--- (2)
                return;

This patch introduces two helpers, put_[un]refcounted_compound_page,
handling the code path (1) and code path (2), respectively.  They both are
tagged __always_inline, thus elmiating function call overhead, making them
operating the same way as before.

They are almost copied verbatim(except one place, a "goto out_put_single"
is expanded), with some comments rephrasing.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>