mm: vmscan: scale number of pages reclaimed by reclaim/compaction based on failures

mm: vmscan: scale number of pages reclaimed by reclaim/compaction based on failures

If allocation fails after compaction then compaction may be deferred for a
number of allocation attempts.  If there are subsequent failures,
compact_defer_shift is increased to defer for longer periods.  This patch
uses that information to scale the number of pages reclaimed with
compact_defer_shift until allocations succeed again.  The rationale is
that reclaiming the normal number of pages still allowed compaction to
fail and its success depends on the number of pages.  If it's failing,
reclaim more pages until it succeeds again.

Note that this is not implying that VM reclaim is not reclaiming enough
pages or that its logic is broken.  try_to_free_pages() always asks for
SWAP_CLUSTER_MAX pages to be reclaimed regardless of order and that is
what it does.  Direct reclaim stops normally with this check.

	if (sc->nr_reclaimed >= sc->nr_to_reclaim)
		goto out;

should_continue_reclaim delays when that check is made until a minimum
number of pages for reclaim/compaction are reclaimed.  It is possible that
this patch could instead set nr_to_reclaim in try_to_free_pages() and
drive it from there but that's behaves differently and not necessarily for
the better.  If driven from do_try_to_free_pages(), it is also possible
that priorities will rise.  When they reach DEF_PRIORITY-2, it will also
start stalling and setting pages for immediate reclaim which is more
disruptive than not desirable in this case.  That is a more wide-reaching
change that could cause another regression related to THP requests causing
interactive jitter.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>