[PATCH 1/9] memcg: make the v1 soft limit knob inert

The v1 soft limit has been deprecated since v6.12 and nobody has
reported depending on it. Start the removal by decoupling the interface
from the implementation: keep memory.soft_limit_in_bytes, but ignore
writes to it and always report the maximum value on read similar to
what memory.kmem.limit_in_bytes already does.

Writes are still parsed, so malformed input keeps returning -EINVAL.
The knob now also behaves the same everywhere: it used to return
-EOPNOTSUPP on PREEMPT_RT, where soft limit reclaim has always been
disabled.

This also fixes the syzbot report linked below. Soft limit reclaim is
the only caller that runs shrink_lruvec() from kswapd against a
specific memcg, so it is the only way to reach lru_gen_shrink_lruvec()
and in turn set_mm_walk(), which warns when called from kswapd.

Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Shakeel Butt <[email protected]>
---
 .../admin-guide/cgroup-v1/memory.rst          | 49 +++----------------
 mm/memcontrol-v1.c                            | 43 +++++++++-------
 2 files changed, 32 insertions(+), 60 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 7db63c002922..7d2a44af52c9 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -47,7 +47,6 @@ Features:
  - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
  - optionally, memory+swap usage can be accounted and limited.
  - hierarchical accounting
- - soft limit
  - moving (recharging) account at moving a task is selectable.
  - usage threshold notifier
  - memory pressure notifier
@@ -76,10 +75,9 @@ Brief summary of control files.
  memory.memsw.failcnt		     show the number of memory+Swap hits limits
  memory.max_usage_in_bytes	     show max memory usage recorded
  memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
- memory.soft_limit_in_bytes	     set/show soft limit of memory usage
-				     This knob is not available on CONFIG_PREEMPT_RT systems.
-                                     This knob is deprecated and shouldn't be
-                                     used.
+ memory.soft_limit_in_bytes	     This knob is deprecated and has no effect.
+                                     Writes are ignored and reads always
+                                     return the maximum value.
  memory.stat			     show various statistics
  memory.use_hierarchy		     set/show hierarchical account enabled
                                      This knob is deprecated and shouldn't be
@@ -340,9 +338,6 @@ memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
 The main "kmem" counter is fed into the main counter, so kmem charges will
 also be visible from the user counter.
 
-Currently no soft limit is implemented for kernel memory. It is future work
-to trigger slab reclaim when those limits are reached.
-
 2.7.1 Current Kernel Memory resources accounted
 -----------------------------------------------
 
@@ -710,42 +705,10 @@ For compatibility reasons writing 1 to memory.use_hierarchy will always pass::
 
 THIS IS DEPRECATED!
 
-Soft limits allow for greater sharing of memory. The idea behind soft limits
-is to allow control groups to use as much of the memory as needed, provided
-
-a. There is no memory contention
-b. They do not exceed their hard limit
-
-When the system detects memory contention or low memory, control groups
-are pushed back to their soft limits. If the soft limit of each control
-group is very high, they are pushed back as much as possible to make
-sure that one control group does not starve the others of memory.
-
-Please note that soft limits is a best-effort feature; it comes with
-no guarantees, but it does its best to make sure that when memory is
-heavily contended for, memory is allocated based on the soft limit
-hints/setup. Currently soft limit based reclaim is set up such that
-it gets invoked from balance_pgdat (kswapd).
-
-7.1 Interface
--------------
-
-Soft limits can be setup by using the following commands (in this example we
-assume a soft limit of 256 MiB)::
-
-	# echo 256M > memory.soft_limit_in_bytes
-
-If we want to change this to 1G, we can at any time use::
+Writing to memory.soft_limit_in_bytes has no effect and reading it will
+always return the maximum value.
 
-	# echo 1G > memory.soft_limit_in_bytes
-
-.. note::
-       Soft limits take effect over a long period of time, since they involve
-       reclaiming memory for balancing between memory cgroups
-
-.. note::
-       It is recommended to set the soft limit always below the hard limit,
-       otherwise the hard limit will take precedence.
+Use memory.low and memory.min in cgroup v2 instead.
 
 .. _cgroup-v1-memory-move-charges:
 
diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index 835fc8e51184..05ef55cae4dc 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -96,7 +96,6 @@ enum {
 	RES_LIMIT,
 	RES_MAX_USAGE,
 	RES_FAILCNT,
-	RES_SOFT_LIMIT,
 };
 
 #ifdef CONFIG_LOCKDEP
@@ -1888,6 +1887,30 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
 	return -EINVAL;
 }
 
+static u64 mem_cgroup_soft_limit_read(struct cgroup_subsys_state *css,
+				      struct cftype *cft)
+{
+	return (u64)PAGE_COUNTER_MAX * PAGE_SIZE;
+}
+
+static ssize_t mem_cgroup_soft_limit_write(struct kernfs_open_file *of,
+					   char *buf, size_t nbytes, loff_t off)
+{
+	unsigned long nr_pages;
+	int ret;
+
+	ret = page_counter_memparse(strstrip(buf), "-1", &nr_pages);
+	if (ret)
+		return ret;
+
+	pr_warn_once("soft_limit_in_bytes is deprecated and will be removed. "
+		     "Writing any value to this file has no effect. "
+		     "Please report your usecase to [email protected] if you "
+		     "depend on this functionality.\n");
+
+	return nbytes;
+}
+
 static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
 			       struct cftype *cft)
 {
@@ -1924,8 +1947,6 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
 		return (u64)counter->watermark * PAGE_SIZE;
 	case RES_FAILCNT:
 		return counter->failcnt;
-	case RES_SOFT_LIMIT:
-		return (u64)READ_ONCE(memcg->soft_limit) * PAGE_SIZE;
 	default:
 		BUG();
 	}
@@ -2020,17 +2041,6 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 			break;
 		}
 		break;
-	case RES_SOFT_LIMIT:
-		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
-			ret = -EOPNOTSUPP;
-		} else {
-			pr_warn_once("soft_limit_in_bytes is deprecated and will be removed. "
-				     "Please report your usecase to [email protected] if you "
-				     "depend on this functionality.\n");
-			WRITE_ONCE(memcg->soft_limit, nr_pages);
-			ret = 0;
-		}
-		break;
 	}
 	return ret ?: nbytes;
 }
@@ -2384,9 +2394,8 @@ struct cftype mem_cgroup_legacy_files[] = {
 	},
 	{
 		.name = "soft_limit_in_bytes",
-		.private = MEMFILE_PRIVATE(_MEM, RES_SOFT_LIMIT),
-		.write = mem_cgroup_write,
-		.read_u64 = mem_cgroup_read_u64,
+		.write = mem_cgroup_soft_limit_write,
+		.read_u64 = mem_cgroup_soft_limit_read,
 	},
 	{
 		.name = "failcnt",
-- 
2.53.0-Meta