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

Ben Hutchings
2014-06-08T02:03:31Z

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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@cmpxchg.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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