[Devel] [PATCH RHEL7 COMMIT] ms/mm: memcontrol: avoid unused function warning

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1160.90.1.vz7.200.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.90.1.vz7.200.4
------>
commit eb4b009d897f769af5688044d0ecaadaa4ecda79
Author: Arnd Bergmann <arnd at arndb.de>
Date:   Wed Jul 5 14:39:52 2023 +0800

    ms/mm: memcontrol: avoid unused function warning
    
    A bugfix in v4.8-rc2 introduced a harmless warning when
    CONFIG_MEMCG_SWAP is disabled but CONFIG_MEMCG is enabled:
    
      mm/memcontrol.c:4085:27: error: 'mem_cgroup_id_get_online' defined but not used [-Werror=unused-function]
       static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
    
    This moves the function inside of the #ifdef block that hides the
    calling function, to avoid the warning.
    
    mFixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
    Link: http://lkml.kernel.org/r/20160824113733.2776701-1-arnd@arndb.de
    Signed-off-by: Arnd Bergmann <arnd at arndb.de>
    Acked-by: Michal Hocko <mhocko at suse.com>
    Acked-by: Vladimir Davydov <vdavydov at virtuozzo.com>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    
    https://jira.vzint.dev/browse/PSBM-147036
    
    (cherry picked from commit 358c07fcc3b60ab08d77f1684de8bd81bcf49a1a)
    Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    
    =================
    Patchset description:
    memcg: release id when offlinging cgroup
    
    We see that container user can deplete memory cgroup ids on the system
    (64k) and prevent further memory cgroup creation. In crash collected by
    our customer in such a situation we see that mem_cgroup_idr is full of
    cgroups from one container with same exact path (cgroup of docker
    service), cgroups are not released because they have kmem charges, this
    kmem charge is for a tmpfs dentry allocated from this cgroup. (And on
    vz7 kernel it seems that such a dentry is only released after umounting
    tmpfs or removing the corresponding file from tmpfs.)
    
    So there is a valid way to hold kmem cgroup for a long time. Similar
    thing was mentioned in mainstream with page cache holding kmem cgroup
    for a long time. And they proposed a way to deal with it - just release
    cgroup id early so that one can allocate new cgroups immediately.
    
    Reproduce:
    https://git.vzint.dev/users/ptikhomirov/repos/helpers/browse/memcg-related/test-mycg-tmpfs.sh
    
    After this fix the number of memory cgroups in /proc/cgroups can now
    show > 64k as we allow to leave memory cgroups hanging while releasing
    their ids.
    
    Note: Maybe it's a bad idea to allow container to eat kernel
    memory with such a hanging cgroups, but yet I don't have better ideas.
    
    https://jira.vzint.dev/browse/PSBM-147473
    https://jira.vzint.dev/browse/PSBM-147036
---
 mm/memcontrol.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index de84dd1ee210..b2911329852f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6685,24 +6685,6 @@ static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 	atomic_add(n, &memcg->id.ref);
 }
 
-static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
-{
-	while (!atomic_inc_not_zero(&memcg->id.ref)) {
-		/*
-		 * The root cgroup cannot be destroyed, so it's refcount must
-		 * always be >= 1.
-		 */
-		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
-			VM_BUG_ON(1);
-			break;
-		}
-		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
-	}
-	return memcg;
-}
-
 static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
@@ -7782,6 +7764,24 @@ static void __init enable_swap_cgroup(void)
 #endif
 
 #ifdef CONFIG_MEMCG_SWAP
+static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
+{
+	while (!atomic_inc_not_zero(&memcg->id.ref)) {
+		/*
+		 * The root cgroup cannot be destroyed, so it's refcount must
+		 * always be >= 1.
+		 */
+		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
+			VM_BUG_ON(1);
+			break;
+		}
+		memcg = parent_mem_cgroup(memcg);
+		if (!memcg)
+			memcg = root_mem_cgroup;
+	}
+	return memcg;
+}
+
 /**
  * mem_cgroup_swapout - transfer a memsw charge to swap
  * @page: page whose memsw charge to transfer