Commit 62376251 authored May 06, 2014 by Johannes Weiner Committed by Linus Torvalds May 06, 2014

revert "mm: vmscan: do not swap anon pages just because free+file is low"



This reverts commit 0bf1457f ("mm: vmscan: do not swap anon pages
just because free+file is low") because it introduced a regression in
mostly-anonymous workloads, where reclaim would become ineffective and
trap every allocating task in direct reclaim.

The problem is that there is a runaway feedback loop in the scan balance
between file and anon, where the balance tips heavily towards a tiny
thrashing file LRU and anonymous pages are no longer being looked at.
The commit in question removed the safe guard that would detect such
situations and respond with forced anonymous reclaim.

This commit was part of a series to fix premature swapping in loads with
relatively little cache, and while it made a small difference, the cure
is obviously worse than the disease.  Revert it.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: <stable@kernel.org>		[3.12+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 6b6751f7

mm/vmscan.c

+18 −0

Original line number	Diff line number	Diff line
		@@ -1915,6 +1915,24 @@ static void get_scan_count(struct lruvec lruvec, struct scan_control sc,
		file = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
		get_lru_size(lruvec, LRU_INACTIVE_FILE);

		/*
		* Prevent the reclaimer from falling into the cache trap: as
		* cache pages start out inactive, every cache fault will tip
		* the scan balance towards the file LRU. And as the file LRU
		* shrinks, so does the window for rotation from references.
		* This means we have a runaway feedback loop where a tiny
		* thrashing file LRU becomes infinitely more attractive than
		* anon pages. Try to detect this based on file LRU size.
		*/
		if (global_reclaim(sc)) {
		unsigned long free = zone_page_state(zone, NR_FREE_PAGES);

		if (unlikely(file + free <= high_wmark_pages(zone))) {
		scan_balance = SCAN_ANON;
		goto out;
		}
		}

		/*
		* There is enough inactive page cache, do not reclaim
		* anything from the anonymous working set right now.

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