mm: simplify reclaim path for MADV_FREE

mm: simplify reclaim path for MADV_FREE

I made reclaim path mess to check and free MADV_FREEed page.  This patch
simplify it with tweaking add_to_swap.

So far, we mark page as PG_dirty when we add the page into swap cache(ie,
add_to_swap) to page out to swap device but this patch moves PG_dirty
marking under try_to_unmap_one when we decide to change pte from anon to
swapent so if any process's pte has swapent for the page, the page must be
swapped out.  IOW, there should be no funcional behavior change.  It makes
relcaim path really simple for MADV_FREE because we just need to check
PG_dirty of page to decide discarding the page or not.

Other thing this patch does is to pass TTU_BATCH_FLUSH to try_to_unmap
when we handle freeable page because I don't see any reason to prevent it.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>