memcg, kmem: deprecate kmem.limit_in_bytes

Cgroup v1 memcg controller has exposed a dedicated kmem limit to users
which turned out to be really a bad idea because there are paths which
cannot shrink the kernel memory usage enough to get below the limit (e.g.
because the accounted memory is not reclaimable).  There are cases when
the failure is even not allowed (e.g.  __GFP_NOFAIL).  This means that the
kmem limit is in excess to the hard limit without any way to shrink and
thus completely useless.  OOM killer cannot be invoked to handle the
situation because that would lead to a premature oom killing.

As a result many places might see ENOMEM returning from kmalloc and result
in unexpected errors.  E.g.  a global OOM killer when there is a lot of
free memory because ENOMEM is translated into VM_FAULT_OOM in #PF path and
therefore pagefault_out_of_memory would result in OOM killer.

Please note that the kernel memory is still accounted to the overall limit
along with the user memory so removing the kmem specific limit should
still allow to contain kernel memory consumption.  Unlike the kmem one,
though, it invokes memory reclaim and targeted memcg oom killing if
necessary.

Start the deprecation process by crying to the kernel log.  Let's see
whether there are relevant usecases and simply return to EINVAL in the
second stage if nobody complains in few releases.

[[email protected]: tweak documentation text]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Thomas Lindroth <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 41bdc038dad97f2963068c4ea229efbc5d144d0e..0ae4f564c2d68dc50ff07e3e590673ce373e3253 100644 (file)
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -85,8 +85,10 @@ 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          set/show hard limit for kernel memory
+                                     This knob is deprecated and shouldn't be
+                                     used. It is planned that this be removed in
+                                     the foreseeable future.
  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 701c1ab79d243a63a24b0cf0bfb502a36889529e..7bb2971dc75e78723a102dc4776f438a3d451c82 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3637,6 +3637,9 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
                        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. "
+                                    "Please report your usecase to [email protected] if you "
+                                    "depend on this functionality.\n");
                        ret = memcg_update_kmem_max(memcg, nr_pages);
                        break;
                case _TCP: