[PATCH] mm, memcg: reconsider kmem.limit_in_bytes deprecation

[Michal Hocko]

This reverts commits 86327e8eb94c ("memcg: drop kmem.limit_in_bytes")
and partially reverts 58056f77502f ("memcg, kmem: further deprecate
kmem.limit_in_bytes") which have incrementally removed support for the
kernel memory accounting hard limit. Unfortunately it has turned out
that there is still userspace depending on the existence of
memory.kmem.limit_in_bytes [1]. The underlying functionality is not
really required but the non-existent file just confuses the userspace
which fails in the result. The patch to fix this on the userspace side
has been submitted but it is hard to predict how it will propagate
through the maze of 3rd party consumers of the software.

Now, reverting alone 86327e8eb94c is not an option because there is
another set of userspace which cannot cope with ENOTSUPP returned when
writing to the file. Therefore we have to go and revisit 58056f77502f
as well. There are two ways to go ahead. Either we give up on the
deprecation and fully revert 58056f77502f as well or we can keep
kmem.limit_in_bytes but make the write a noop and warn about the fact.
This should work for both known breaking workloads which depend on the
existence but do not depend on the hard limit enforcement.

[1] http://lkml.kernel.org/r/20230920081101.GA12096@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 86327e8eb94c ("memcg: drop kmem.limit_in_bytes")
Fixes: 58056f77502f ("memcg, kmem: further deprecate kmem.limit_in_bytes")
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 Documentation/admin-guide/cgroup-v1/memory.rst |  7 +++++++
 mm/memcontrol.c                                | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 5f502bf68fbc..ff456871bf4b 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -92,6 +92,13 @@ Brief summary of control files.
  memory.oom_control		     set/show oom controls.
  memory.numa_stat		     show the number of memory usage per numa
 				     node
+ memory.kmem.limit_in_bytes          Deprecated knob to set and read the kernel
+                                     memory hard limit. Kernel hard limit is not
+                                     supported since 5.16. Writing any value to
+                                     do file will not have any effect same as if
+                                     nokmem kernel parameter was specified.
+                                     Kernel memory is still charged and reported
+                                     by memory.kmem.usage_in_bytes.
  memory.kmem.usage_in_bytes          show current kernel memory allocation
  memory.kmem.failcnt                 show the number of kernel memory usage
 				     hits limits
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a4d3282493b6..ac7f14b2338d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3097,6 +3097,7 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 				   unsigned int nr_pages)
 {
+	struct page_counter *counter;
 	struct mem_cgroup *memcg;
 	int ret;
 
@@ -3107,6 +3108,10 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 		goto out;
 
 	memcg_account_kmem(memcg, nr_pages);
+
+	/* There is no way to set up kmem hard limit so this operation cannot fail */
+	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+		WARN_ON(!page_counter_try_charge(&memcg->kmem, nr_pages, &counter));
 out:
 	css_put(&memcg->css);
 
@@ -3867,6 +3872,13 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 		case _MEMSWAP:
 			ret = mem_cgroup_resize_max(memcg, nr_pages, true);
 			break;
+		case _KMEM:
+			pr_warn_once("kmem.limit_in_bytes is deprecated and will be removed. "
+				     "Writing any value to this file has no effect. "
+				     "Please report your usecase to linux-mm@xxxxxxxxx if you "
+				     "depend on this functionality.\n");
+			ret = 0;
+			break;
 		case _TCP:
 			ret = memcg_update_tcp_max(memcg, nr_pages);
 			break;
-- 
2.30.2

-- 
Michal Hocko
SUSE Labs