The new memory.min limit is similar to memory.low, just no bypassing it
when reclaim is desparate.  Prefer oom kills before reclaim memory below
memory.min.  Sharing more code with memory_cgroup_low() is possible, but
the idea is posted here for simplicity.

Signed-off-by: Greg Thelen <gthe...@google.com>
---
 include/linux/memcontrol.h |  8 ++++++
 mm/memcontrol.c            | 58 ++++++++++++++++++++++++++++++++++++++
 mm/vmscan.c                |  3 ++
 3 files changed, 69 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index c46016bb25eb..22bb4a88653a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -178,6 +178,7 @@ struct mem_cgroup {
        struct page_counter tcpmem;
 
        /* Normal memory consumption range */
+       unsigned long min;
        unsigned long low;
        unsigned long high;
 
@@ -281,6 +282,7 @@ static inline bool mem_cgroup_disabled(void)
        return !cgroup_subsys_enabled(memory_cgrp_subsys);
 }
 
+bool mem_cgroup_min(struct mem_cgroup *root, struct mem_cgroup *memcg);
 bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg);
 
 int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
@@ -726,6 +728,12 @@ static inline void mem_cgroup_event(struct mem_cgroup 
*memcg,
 {
 }
 
+static inline bool mem_cgroup_min(struct mem_cgroup *root,
+                                 struct mem_cgroup *memcg)
+{
+       return false;
+}
+
 static inline bool mem_cgroup_low(struct mem_cgroup *root,
                                  struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9668f620203a..b2aaed4003b4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5058,6 +5058,36 @@ static u64 memory_current_read(struct 
cgroup_subsys_state *css,
        return (u64)page_counter_read(&memcg->memory) * PAGE_SIZE;
 }
 
+static int memory_min_show(struct seq_file *m, void *v)
+{
+       struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
+       unsigned long min = READ_ONCE(memcg->min);
+
+       if (min == PAGE_COUNTER_MAX)
+               seq_puts(m, "max\n");
+       else
+               seq_printf(m, "%llu\n", (u64)min * PAGE_SIZE);
+
+       return 0;
+}
+
+static ssize_t memory_min_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 min;
+       int err;
+
+       buf = strstrip(buf);
+       err = page_counter_memparse(buf, "max", &min);
+       if (err)
+               return err;
+
+       memcg->min = min;
+
+       return nbytes;
+}
+
 static int memory_low_show(struct seq_file *m, void *v)
 {
        struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
@@ -5288,6 +5318,12 @@ static struct cftype memory_files[] = {
                .flags = CFTYPE_NOT_ON_ROOT,
                .read_u64 = memory_current_read,
        },
+       {
+               .name = "min",
+               .flags = CFTYPE_NOT_ON_ROOT,
+               .seq_show = memory_min_show,
+               .write = memory_min_write,
+       },
        {
                .name = "low",
                .flags = CFTYPE_NOT_ON_ROOT,
@@ -5336,6 +5372,28 @@ struct cgroup_subsys memory_cgrp_subsys = {
        .early_init = 0,
 };
 
+/**
+ * mem_cgroup_min returns true for a memcg below its min limit.  Such memcg are
+ * excempt from reclaim.
+ */
+bool mem_cgroup_min(struct mem_cgroup *root, struct mem_cgroup *memcg)
+{
+       if (mem_cgroup_disabled())
+               return false;
+
+       if (!root)
+               root = root_mem_cgroup;
+
+       if (memcg == root)
+               return false;
+
+       for (; memcg != root; memcg = parent_mem_cgroup(memcg)) {
+               if (page_counter_read(&memcg->memory) <= memcg->min)
+                       return true; /* protect */
+       }
+       return false; /* !protect */
+}
+
 /**
  * mem_cgroup_low - check if memory consumption is below the normal range
  * @root: the top ancestor of the sub-tree being checked
diff --git a/mm/vmscan.c b/mm/vmscan.c
index cd5dc3faaa57..15ae19a38ad5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2539,6 +2539,9 @@ static bool shrink_node(pg_data_t *pgdat, struct 
scan_control *sc)
                        unsigned long reclaimed;
                        unsigned long scanned;
 
+                       if (mem_cgroup_min(root, memcg))
+                               continue;
+
                        if (mem_cgroup_low(root, memcg)) {
                                if (!sc->memcg_low_reclaim) {
                                        sc->memcg_low_skipped = 1;
-- 
2.17.0.484.g0c8726318c-goog

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