+ vmscan-prevent-background-aging-of-anon-page-in-no-swap-system.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     vmscan: prevent background aging of anon page in no swap system
has been added to the -mm tree.  Its filename is
     vmscan-prevent-background-aging-of-anon-page-in-no-swap-system.patch

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------------------------------------------------------
Subject: vmscan: prevent background aging of anon page in no swap system
From: Minchan Kim <minchan.kim@gmail.com>

Ying Han reported that backing aging of anon pages in no swap system
causes unnecessary TLB flush.

When I sent a patch(69c8548175), I wanted this patch but Rik pointed out
and allowed aging of anon pages to give a chance to promote from inactive
to active LRU.

It has a two problem.

1) non-swap system

Never make sense to age anon pages.

2) swap configured but still doesn't swapon

It doesn't make sense to age anon pages until swap-on time.  But it's
arguable.  If we have aged anon pages by swapon, VM have moved anon pages
from active to inactive.  And in the time swapon by admin, the VM can't
reclaim hot pages so we can protect hot pages swapout.

But let's think about it.  When does swap-on happen?  It depends on admin.
 we can't expect it.  Nonetheless, we have done aging of anon pages to
protect hot pages swapout.  It means we lost run time overhead when below
high watermark but gain hot page swap-[in/out] overhead when VM decide
swapout.  Is it true?  Let's think more detail.  We don't promote anon
pages in case of non-swap system.  So even though VM does aging of anon
pages, the pages would be in inactive LRU for a long time.  It means many
of pages in there would mark access bit again.  So access bit hot/code
separation would be pointless.

This patch prevents unnecessary anon pages demotion in not-swapon and
non-configured swap system.  Of course, it could make side effect that hot
anon pages could swap out when admin does swap on.  But I think sooner or
later it would be steady state.  So it's not a big problem.  We could lose
someting but gain more thing(TLB flush and unnecessary function call to
demote anon pages).

I used total_swap_pages because we want to age anon pages even though swap
full happens.

Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN mm/vmscan.c~vmscan-prevent-background-aging-of-anon-page-in-no-swap-system mm/vmscan.c
--- a/mm/vmscan.c~vmscan-prevent-background-aging-of-anon-page-in-no-swap-system
+++ a/mm/vmscan.c
@@ -2137,7 +2137,7 @@ loop_again:
 			 * Do some background aging of the anon list, to give
 			 * pages a chance to be referenced before reclaiming.
 			 */
-			if (inactive_anon_is_low(zone, &sc))
+			if (total_swap_pages > 0 && inactive_anon_is_low(zone, &sc))
 				shrink_active_list(SWAP_CLUSTER_MAX, zone,
 							&sc, priority, 0);
 
_

Patches currently in -mm which might be from minchan.kim@gmail.com are

vmscan-prevent-background-aging-of-anon-page-in-no-swap-system.patch
mm-vmap-area-cache.patch