linux-kernel - Re: [PATCH 3.14 103/228] revert "mm: vmscan: do not swap anon pages just because free+file is low"

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Message-ID: <1402192995.23860.21.camel@deadeye.wl.decadent.org.uk>
Date:	Sun, 08 Jun 2014 03:03:15 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Johannes Weiner <hannes@...xchg.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Rafael Aquini <aquini@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3.14 103/228] revert "mm: vmscan: do not swap anon pages
 just because free+file is low"

On Wed, 2014-06-04 at 16:22 -0700, Greg Kroah-Hartman wrote:
> 3.14-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Johannes Weiner <hannes@...xchg.org>
> 
> commit 623762517e2370be3b3f95f4fe08d6c063a49b06 upstream.
> 
> This reverts commit 0bf1457f0cfc ("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.
[...]

That commit was not included in 3.14 or in subsequent stable updates, so
this 'revert' is not approriate.  We now have duplicate checks:

	/*
	 * If it's foreseeable that reclaiming the file cache won't be
	 * enough to get the zone back into a desirable shape, we have
	 * to swap.  Better start now and leave the - probably heavily
	 * thrashing - remaining file pages alone.
	 */
	if (global_reclaim(sc)) {
		free = zone_page_state(zone, NR_FREE_PAGES);
		if (unlikely(file + free <= high_wmark_pages(zone))) {
			scan_balance = SCAN_ANON;
			goto out;
		}
	}

	/*
	 * 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;
		}
	}

Ben.

-- 
Ben Hutchings
Never attribute to conspiracy what can adequately be explained by stupidity.

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