Re: [PATCH v3 2/3] mm/memcg: Simplify mem_cgroup_get_max()

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On 9/14/20 10:47 AM, Michal Hocko wrote:
On Mon 14-09-20 10:19:55, Waiman Long wrote:
The mem_cgroup_get_max() function used to get memory+swap max from
both the v1 memsw and v2 memory+swap page counters & return the maximum
of these 2 values. This is redundant and it is more efficient to just
get either the v1 or the v2 values depending on which one is currently
in use.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
  mm/memcontrol.c | 24 +++++++++++++-----------
  1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8c74f1200261..2331d4bc7c4d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1633,17 +1633,19 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
   */
  unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
  {
-	unsigned long max;
-
-	max = READ_ONCE(memcg->memory.max);
-	if (mem_cgroup_swappiness(memcg)) {
-		unsigned long memsw_max;
-		unsigned long swap_max;
-
-		memsw_max = memcg->memsw.max;
-		swap_max = READ_ONCE(memcg->swap.max);
-		swap_max = min(swap_max, (unsigned long)total_swap_pages);
-		max = min(max + swap_max, memsw_max);
+	unsigned long max = READ_ONCE(memcg->memory.max);
+
+	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
+		if (mem_cgroup_swappiness(memcg))
+			max += min(READ_ONCE(memcg->swap.max),
+				   (unsigned long)total_swap_pages);
+	} else { /* v1 */
+		if (mem_cgroup_swappiness(memcg)) {
+			unsigned long memsw = READ_ONCE(memcg->memsw.max);
+
+			if (memsw > max)
+				max += min(memsw - max, (unsigned long)total_swap_pages);
Yes this looks better. But, memsw can never be smaller than the hard
limit (mem_cgroup_resize_max). I would find it slightly easier to
understand if you did
	/* calculate swap excess capacity from memsw limit*/
	unsigned long memsw = READ_ONCE(memcg->memsw.max) - max;
	max += min (memsw, total_swap_pages);

Right, I thought it was possible to set memsw lower than mem. It was not allowed. So the extra check is unnecessary. Will fix that.

Cheers,
Longman





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