memcg: deprecate memory.force_empty knob

force_empty has been introduced primarily to drop memory before it gets
reparented on the group removal.  This alone doesn't sound fully justified
because reparented pages which are not in use can be reclaimed also later
when there is a memory pressure on the parent level.

Mark the knob CFTYPE_INSANE which tells the cgroup core that it shouldn't
create the knob with the experimental sane_behavior.  Other users will get
informed about the deprecation and asked to tell us more because I do not
expect most users will use sane_behavior cgroups mode very soon.

Anyway I expect that most users will be simply cgroup remove handlers
which do that since ever without having any good reason for it.

If somebody really cares because reparented pages, which would be dropped
otherwise, push out more important ones then we should fix the reparenting
code and put pages to the tail.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index c564882f49b19caf76e74b73fe1f05f20a1d4637..06e756434f5884cbe521f1c66d6fa793d729ab55 100644 (file)
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -477,6 +477,9 @@ About use_hierarchy, see Section 6.
   write will still return success. In this case, it is expected that
   memory.kmem.usage_in_bytes == memory.usage_in_bytes.
 
+  Please note that this knob is considered deprecated and will be removed
+  in future.
+
   About use_hierarchy, see Section 6.
 
 5.2 stat file

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 38d713a7addb416c0734311d5d15bd3d1b1a98ab..7bd7bbf11f90601746554468bffc044f0348ee90 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4797,6 +4797,10 @@ static ssize_t mem_cgroup_force_empty_write(struct kernfs_open_file *of,
 
        if (mem_cgroup_is_root(memcg))
                return -EINVAL;
+       pr_info("%s (%d): memory.force_empty is deprecated and will be removed.",
+                       current->comm, task_pid_nr(current));
+       pr_cont(" Let us know if you know if it needed in your usecase at");
+       pr_cont(" linux-mm@kvack.org\n");
        return mem_cgroup_force_empty(memcg) ?: nbytes;
 }
 
@@ -6047,6 +6051,7 @@ static struct cftype mem_cgroup_files[] = {
        },
        {
                .name = "force_empty",
+               .flags = CFTYPE_INSANE,
                .write = mem_cgroup_force_empty_write,
        },
        {