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Subject[PATCH 2/2] mm: memcontrol: try harder to set a new memory.high
Date
Setting a memory.high limit below the usage makes almost no effort to
shrink the cgroup to the new target size.

While memory.high is a "soft" limit that isn't supposed to cause OOM
situations, we should still try harder to meet a user request through
persistent reclaim.

For example, after setting a 10M memory.high on an 800M cgroup full of
file cache, the usage shrinks to about 350M:

+ cat /cgroup/workingset/memory.current
841568256
+ echo 10M
+ cat /cgroup/workingset/memory.current
355729408

This isn't exactly what the user would expect to happen. Setting the
value a few more times eventually whittles the usage down to what we
are asking for:

+ echo 10M
+ cat /cgroup/workingset/memory.current
104181760
+ echo 10M
+ cat /cgroup/workingset/memory.current
31801344
+ echo 10M
+ cat /cgroup/workingset/memory.current
10440704

To improve this, add reclaim retry loops to the memory.high write()
callback, similar to what we do for memory.max, to make a reasonable
effort that the usage meets the requested size after the call returns.

Afterwards, a single write() to memory.high is enough in all but
extreme cases:

+ cat /cgroup/workingset/memory.current
841609216
+ echo 10M
+ cat /cgroup/workingset/memory.current
10182656

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/memcontrol.c | 30 ++++++++++++++++++++++++------
1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ff90d4e7df37..8090b4c99ac7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6074,7 +6074,8 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
char *buf, size_t nbytes, loff_t off)
{
struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
- unsigned long nr_pages;
+ unsigned int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
+ bool drained = false;
unsigned long high;
int err;

@@ -6085,12 +6086,29 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,

memcg->high = high;

- nr_pages = page_counter_read(&memcg->memory);
- if (nr_pages > high)
- try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
- GFP_KERNEL, true);
+ for (;;) {
+ unsigned long nr_pages = page_counter_read(&memcg->memory);
+ unsigned long reclaimed;
+
+ if (nr_pages <= high)
+ break;
+
+ if (signal_pending(current))
+ break;
+
+ if (!drained) {
+ drain_all_stock(memcg);
+ drained = true;
+ continue;
+ }
+
+ reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
+ GFP_KERNEL, true);
+
+ if (!reclaimed && !nr_retries--)
+ break;
+ }

- memcg_wb_domain_size_changed(memcg);
return nbytes;
}

--
2.23.0
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