Commit be09102b authored Jun 07, 2018 by Tejun Heo Committed by Linus Torvalds Jun 07, 2018

mm: memcg: allow lowering memory.swap.max below the current usage

Currently an attempt to set swap.max into a value lower than the actual
swap usage fails, which causes configuration problems as there's no way
of lowering the configuration below the current usage short of turning
off swap entirely.  This makes swap.max difficult to use and allows
delegatees to lock the delegator out of reducing swap allocation.

This patch updates swap_max_write() so that the limit can be lowered
below the current usage.  It doesn't implement active reclaiming of swap
entries for the following reasons.

* mem_cgroup_swap_full() already tells the swap machinary to
  aggressively reclaim swap entries if the usage is above 50% of
  limit, so simply lowering the limit automatically triggers gradual
  reclaim.

* Forcing back swapped out pages is likely to heavily impact the
  workload and mess up the working set.  Given that swap usually is a
  lot less valuable and less scarce, letting the existing usage
  dissipate over time through the above gradual reclaim and as they're
  falted back in is likely the better behavior.

Link: http://lkml.kernel.org/r/20180523185041.GR1718769@devbig577.frc2.facebook.com


Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Rik van Riel <riel@surriel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shaohua Li <shli@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 20acce67

Documentation/admin-guide/cgroup-v2.rst

+5 −0

Original line number	Diff line number	Diff line
		@@ -1245,6 +1245,11 @@ PAGE_SIZE multiple when read back.
		because of running out of swap system-wide or max
		limit.

		When reduced under the current usage, the existing swap
		entries are reclaimed gradually and the swap usage may stay
		higher than the limit for an extended period of time. This
		reduces the impact on the workload and memory management.


		Usage Guidelines
		~~~~~~~~~~~~~~~~

mm/memcontrol.c

+1 −5

Original line number	Diff line number	Diff line
		@@ -6280,11 +6280,7 @@ static ssize_t swap_max_write(struct kernfs_open_file *of,
		if (err)
		return err;

		mutex_lock(&memcg_max_mutex);
		err = page_counter_set_max(&memcg->swap, max);
		mutex_unlock(&memcg_max_mutex);
		if (err)
		return err;
		xchg(&memcg->swap.max, max);

		return nbytes;
		}

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