| Message ID | 20240724190214.1108049-2-kinseyho@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Improve mem_cgroup_iter() | expand |
Hi Kinsey, kernel test robot noticed the following build warnings: [auto build test WARNING on akpm-mm/mm-everything] [also build test WARNING on linus/master v6.10 next-20240725] [If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in https://git-scm.com/docs/git-format-patch#_base_tree_information] url: https://github.com/intel-lab-lkp/linux/commits/Kinsey-Ho/mm-don-t-hold-css-refcnt-during-traversal/20240725-030750 base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything patch link: https://lore.kernel.org/r/20240724190214.1108049-2-kinseyho%40google.com patch subject: [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during traversal config: x86_64-randconfig-121-20240725 (https://download.01.org/0day-ci/archive/20240726/202407260047.sIuRFLJE-lkp@intel.com/config) compiler: gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0 reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240726/202407260047.sIuRFLJE-lkp@intel.com/reproduce) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp@intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202407260047.sIuRFLJE-lkp@intel.com/ sparse warnings: (new ones prefixed by >>) >> mm/memcontrol.c:1063:23: sparse: sparse: incorrect type in initializer (different address spaces) @@ expected struct mem_cgroup [noderef] __rcu *__old @@ got struct mem_cgroup *[assigned] pos @@ mm/memcontrol.c:1063:23: sparse: expected struct mem_cgroup [noderef] __rcu *__old mm/memcontrol.c:1063:23: sparse: got struct mem_cgroup *[assigned] pos mm/memcontrol.c:1063:23: sparse: sparse: incorrect type in initializer (different address spaces) @@ expected struct mem_cgroup [noderef] __rcu *__new @@ got struct mem_cgroup *[assigned] memcg @@ mm/memcontrol.c:1063:23: sparse: expected struct mem_cgroup [noderef] __rcu *__new mm/memcontrol.c:1063:23: sparse: got struct mem_cgroup *[assigned] memcg >> mm/memcontrol.c:1101:17: sparse: sparse: incorrect type in initializer (different address spaces) @@ expected struct mem_cgroup [noderef] __rcu *__old @@ got struct mem_cgroup *dead_memcg @@ mm/memcontrol.c:1101:17: sparse: expected struct mem_cgroup [noderef] __rcu *__old mm/memcontrol.c:1101:17: sparse: got struct mem_cgroup *dead_memcg mm/memcontrol.c: note: in included file (through include/linux/cgroup-defs.h): include/linux/list.h:83:21: sparse: sparse: self-comparison always evaluates to true mm/memcontrol.c: note: in included file: include/linux/memcontrol.h:747:9: sparse: sparse: context imbalance in 'folio_lruvec_lock' - wrong count at exit include/linux/memcontrol.h:747:9: sparse: sparse: context imbalance in 'folio_lruvec_lock_irq' - wrong count at exit include/linux/memcontrol.h:747:9: sparse: sparse: context imbalance in 'folio_lruvec_lock_irqsave' - wrong count at exit mm/memcontrol.c: note: in included file (through include/linux/cgroup-defs.h): include/linux/list.h:83:21: sparse: sparse: self-comparison always evaluates to true vim +1063 mm/memcontrol.c 4b569387c0d566 Nhat Pham 2023-10-06 974 5660048ccac873 Johannes Weiner 2012-01-12 975 /** 5660048ccac873 Johannes Weiner 2012-01-12 976 * mem_cgroup_iter - iterate over memory cgroup hierarchy 5660048ccac873 Johannes Weiner 2012-01-12 977 * @root: hierarchy root 5660048ccac873 Johannes Weiner 2012-01-12 978 * @prev: previously returned memcg, NULL on first invocation 5660048ccac873 Johannes Weiner 2012-01-12 979 * @reclaim: cookie for shared reclaim walks, NULL for full walks 5660048ccac873 Johannes Weiner 2012-01-12 980 * 5660048ccac873 Johannes Weiner 2012-01-12 981 * Returns references to children of the hierarchy below @root, or 5660048ccac873 Johannes Weiner 2012-01-12 982 * @root itself, or %NULL after a full round-trip. 5660048ccac873 Johannes Weiner 2012-01-12 983 * 5660048ccac873 Johannes Weiner 2012-01-12 984 * Caller must pass the return value in @prev on subsequent 5660048ccac873 Johannes Weiner 2012-01-12 985 * invocations for reference counting, or use mem_cgroup_iter_break() 5660048ccac873 Johannes Weiner 2012-01-12 986 * to cancel a hierarchy walk before the round-trip is complete. 5660048ccac873 Johannes Weiner 2012-01-12 987 * 05bdc520b3ad39 Miaohe Lin 2020-10-13 988 * Reclaimers can specify a node in @reclaim to divide up the memcgs 05bdc520b3ad39 Miaohe Lin 2020-10-13 989 * in the hierarchy among all concurrent reclaimers operating on the 05bdc520b3ad39 Miaohe Lin 2020-10-13 990 * same node. 5660048ccac873 Johannes Weiner 2012-01-12 991 */ 694fbc0fe78518 Andrew Morton 2013-09-24 992 struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, 9f3a0d0933de07 Johannes Weiner 2012-01-12 993 struct mem_cgroup *prev, 694fbc0fe78518 Andrew Morton 2013-09-24 994 struct mem_cgroup_reclaim_cookie *reclaim) 7d74b06f240f1b KAMEZAWA Hiroyuki 2010-10-27 995 { 3f649ab728cda8 Kees Cook 2020-06-03 996 struct mem_cgroup_reclaim_iter *iter; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 997 struct cgroup_subsys_state *css = NULL; 9f3a0d0933de07 Johannes Weiner 2012-01-12 998 struct mem_cgroup *memcg = NULL; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 999 struct mem_cgroup *pos = NULL; 711d3d2c9bc3fb KAMEZAWA Hiroyuki 2010-10-27 1000 694fbc0fe78518 Andrew Morton 2013-09-24 1001 if (mem_cgroup_disabled()) 694fbc0fe78518 Andrew Morton 2013-09-24 1002 return NULL; 5660048ccac873 Johannes Weiner 2012-01-12 1003 9f3a0d0933de07 Johannes Weiner 2012-01-12 1004 if (!root) 9f3a0d0933de07 Johannes Weiner 2012-01-12 1005 root = root_mem_cgroup; 9f3a0d0933de07 Johannes Weiner 2012-01-12 1006 542f85f9ae4acd Michal Hocko 2013-04-29 1007 rcu_read_lock(); 14067bb3e24b96 KAMEZAWA Hiroyuki 2009-04-02 1008 527a5ec9a53471 Johannes Weiner 2012-01-12 1009 if (reclaim) { ef8f2327996b5c Mel Gorman 2016-07-28 1010 struct mem_cgroup_per_node *mz; 527a5ec9a53471 Johannes Weiner 2012-01-12 1011 a3747b53b1771a Johannes Weiner 2021-04-29 1012 mz = root->nodeinfo[reclaim->pgdat->node_id]; 9da83f3fc74b80 Yafang Shao 2019-11-30 1013 iter = &mz->iter; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1014 a9320aae68a1cd Wei Yang 2022-04-28 1015 /* a9320aae68a1cd Wei Yang 2022-04-28 1016 * On start, join the current reclaim iteration cycle. a9320aae68a1cd Wei Yang 2022-04-28 1017 * Exit when a concurrent walker completes it. a9320aae68a1cd Wei Yang 2022-04-28 1018 */ a9320aae68a1cd Wei Yang 2022-04-28 1019 if (!prev) a9320aae68a1cd Wei Yang 2022-04-28 1020 reclaim->generation = iter->generation; a9320aae68a1cd Wei Yang 2022-04-28 1021 else if (reclaim->generation != iter->generation) 542f85f9ae4acd Michal Hocko 2013-04-29 1022 goto out_unlock; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1023 fe6faac115aa89 Kinsey Ho 2024-07-24 1024 pos = rcu_dereference(iter->position); 89d8330ccf2ad4 Wei Yang 2022-04-28 1025 } else if (prev) { 89d8330ccf2ad4 Wei Yang 2022-04-28 1026 pos = prev; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1027 } 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1028 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1029 if (pos) 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1030 css = &pos->css; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1031 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1032 for (;;) { 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1033 css = css_next_descendant_pre(css, &root->css); 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1034 if (!css) { 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1035 /* 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1036 * Reclaimers share the hierarchy walk, and a 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1037 * new one might jump in right at the end of 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1038 * the hierarchy - make sure they see at least 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1039 * one group and restart from the beginning. 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1040 */ 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1041 if (!prev) 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1042 continue; 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1043 break; 542f85f9ae4acd Michal Hocko 2013-04-29 1044 } 5f578161971863 Michal Hocko 2013-04-29 1045 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1046 /* 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1047 * Verify the css and acquire a reference. The root 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1048 * is provided by the caller, so we know it's alive 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1049 * and kicking, and don't take an extra reference. 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1050 */ 41555dadbff8d2 Wei Yang 2022-04-28 1051 if (css == &root->css || css_tryget(css)) { 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1052 memcg = mem_cgroup_from_css(css); 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1053 break; 41555dadbff8d2 Wei Yang 2022-04-28 1054 } 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1055 } 9f3a0d0933de07 Johannes Weiner 2012-01-12 1056 527a5ec9a53471 Johannes Weiner 2012-01-12 1057 if (reclaim) { 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1058 /* 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1059 * The position could have already been updated by a competing 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1060 * thread, so check that the value hasn't changed since we read 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1061 * it to avoid reclaiming from the same cgroup twice. 5ac8fb31ad2ebd Johannes Weiner 2014-12-10 1062 */ 6df38689e0e9a0 Vladimir Davydov 2015-12-29 @1063 (void)cmpxchg(&iter->position, pos, memcg); 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1064 19f39402864ea3 Michal Hocko 2013-04-29 1065 if (!memcg) 527a5ec9a53471 Johannes Weiner 2012-01-12 1066 iter->generation++; 527a5ec9a53471 Johannes Weiner 2012-01-12 1067 } 14067bb3e24b96 KAMEZAWA Hiroyuki 2009-04-02 1068 542f85f9ae4acd Michal Hocko 2013-04-29 1069 out_unlock: 542f85f9ae4acd Michal Hocko 2013-04-29 1070 rcu_read_unlock(); c40046f3ad5e87 Michal Hocko 2013-04-29 1071 if (prev && prev != root) c40046f3ad5e87 Michal Hocko 2013-04-29 1072 css_put(&prev->css); c40046f3ad5e87 Michal Hocko 2013-04-29 1073 9f3a0d0933de07 Johannes Weiner 2012-01-12 1074 return memcg; 9f3a0d0933de07 Johannes Weiner 2012-01-12 1075 } 14067bb3e24b96 KAMEZAWA Hiroyuki 2009-04-02 1076 5660048ccac873 Johannes Weiner 2012-01-12 1077 /** 5660048ccac873 Johannes Weiner 2012-01-12 1078 * mem_cgroup_iter_break - abort a hierarchy walk prematurely 5660048ccac873 Johannes Weiner 2012-01-12 1079 * @root: hierarchy root 5660048ccac873 Johannes Weiner 2012-01-12 1080 * @prev: last visited hierarchy member as returned by mem_cgroup_iter() 5660048ccac873 Johannes Weiner 2012-01-12 1081 */ 5660048ccac873 Johannes Weiner 2012-01-12 1082 void mem_cgroup_iter_break(struct mem_cgroup *root, 9f3a0d0933de07 Johannes Weiner 2012-01-12 1083 struct mem_cgroup *prev) 9f3a0d0933de07 Johannes Weiner 2012-01-12 1084 { 711d3d2c9bc3fb KAMEZAWA Hiroyuki 2010-10-27 1085 if (!root) 711d3d2c9bc3fb KAMEZAWA Hiroyuki 2010-10-27 1086 root = root_mem_cgroup; 9f3a0d0933de07 Johannes Weiner 2012-01-12 1087 if (prev && prev != root) 9f3a0d0933de07 Johannes Weiner 2012-01-12 1088 css_put(&prev->css); 14067bb3e24b96 KAMEZAWA Hiroyuki 2009-04-02 1089 } 9f3a0d0933de07 Johannes Weiner 2012-01-12 1090 54a83d6bcbf8f4 Miles Chen 2019-08-13 1091 static void __invalidate_reclaim_iterators(struct mem_cgroup *from, 54a83d6bcbf8f4 Miles Chen 2019-08-13 1092 struct mem_cgroup *dead_memcg) 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1093 { 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1094 struct mem_cgroup_reclaim_iter *iter; ef8f2327996b5c Mel Gorman 2016-07-28 1095 struct mem_cgroup_per_node *mz; ef8f2327996b5c Mel Gorman 2016-07-28 1096 int nid; 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1097 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1098 for_each_node(nid) { a3747b53b1771a Johannes Weiner 2021-04-29 1099 mz = from->nodeinfo[nid]; 9da83f3fc74b80 Yafang Shao 2019-11-30 1100 iter = &mz->iter; 9da83f3fc74b80 Yafang Shao 2019-11-30 @1101 cmpxchg(&iter->position, dead_memcg, NULL); 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1102 } 6df38689e0e9a0 Vladimir Davydov 2015-12-29 1103 } 54a83d6bcbf8f4 Miles Chen 2019-08-13 1104
On Wed, Jul 24, 2024 at 07:02:11PM +0000, Kinsey Ho wrote: > To obtain the pointer to the saved memcg position, mem_cgroup_iter() > currently holds css->refcnt during memcg traversal only to put > css->refcnt at the end of the routine. This isn't necessary as an > rcu_read_lock is already held throughout the function. > > Remove css->refcnt usage during traversal by leveraging RCU. Eh, I don't know about this. RCU ensures that the css memory isn't freed. The tryget ensures that the css is still alive and valid. In this case, it just so happens that the sibling linkage is also rcu protected. But accessing random css members when the refcount is 0 is kind of sketchy. On the other hand, the refcount is guaranteed to be valid, and rcu + tryget is a common pattern. What does this buy us? The tryget is cheap.
On Thu, Jul 25, 2024 at 04:43:46PM -0400, Johannes Weiner wrote: > On Wed, Jul 24, 2024 at 07:02:11PM +0000, Kinsey Ho wrote: > > To obtain the pointer to the saved memcg position, mem_cgroup_iter() > > currently holds css->refcnt during memcg traversal only to put > > css->refcnt at the end of the routine. This isn't necessary as an > > rcu_read_lock is already held throughout the function. > > > > Remove css->refcnt usage during traversal by leveraging RCU. > > Eh, I don't know about this. > > RCU ensures that the css memory isn't freed. > > The tryget ensures that the css is still alive and valid. > > In this case, it just so happens that the sibling linkage is also rcu > protected. But accessing random css members when the refcount is 0 is > kind of sketchy. On the other hand, the refcount is guaranteed to be > valid, and rcu + tryget is a common pattern. I also spent quite some time thinking about potential bad consequences, but _it seems_ to be safe (but I agree it feels dangerous). > > What does this buy us? The tryget is cheap. To be fair, tryget is not always cheap. Offline/dying cgroups have an atomic operation there.
On Thu, Jul 25, 2024 at 1:43 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Wed, Jul 24, 2024 at 07:02:11PM +0000, Kinsey Ho wrote: > > To obtain the pointer to the saved memcg position, mem_cgroup_iter() > > currently holds css->refcnt during memcg traversal only to put > > css->refcnt at the end of the routine. This isn't necessary as an > > rcu_read_lock is already held throughout the function. > > > > Remove css->refcnt usage during traversal by leveraging RCU. > > Eh, I don't know about this. > > RCU ensures that the css memory isn't freed. > > The tryget ensures that the css is still alive and valid. > > In this case, it just so happens that the sibling linkage is also rcu > protected. But accessing random css members when the refcount is 0 is > kind of sketchy. On the other hand, the refcount is guaranteed to be > valid, and rcu + tryget is a common pattern. To be fair, the documentation of css_next_descendant_pre() mentions that the requirements are: - Either cgroup_mutex or RCU lock is held. - Both @pos and @root are accessible. - @pos is a descendant of @root. This reads to me like it is intentional that RCU protection is enough for @pos and @root, and that the sibling linkage is RCU protected by design. Perhaps we could clarify this further (whether at css_next_descendant_pre(), or above the definition of the linkage members). > > What does this buy us? The tryget is cheap. mem_cgroup_iter() is not an easy function to follow, so I personally appreciate the simplicity gains tbh.
Thank you Johannes, Roman, and Yosry for reviewing this patch! On Thu, Jul 25, 2024 at 3:34 PM Yosry Ahmed <yosryahmed@google.com> wrote: > On Thu, Jul 25, 2024 at 1:43 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > What does this buy us? The tryget is cheap. > > mem_cgroup_iter() is not an easy function to follow, so I personally > appreciate the simplicity gains tbh. Yes, the main intention here was to simplify the code’s readability. > This reads to me like it is intentional that RCU protection is enough > for @pos and @root, and that the sibling linkage is RCU protected by > design. Perhaps we could clarify this further (whether at > css_next_descendant_pre(), or above the definition of the linkage > members). Do we want to move forward with Yosry’s suggestion to clarify that the sibling linkage is RCU-protected by design? Perhaps this clarification can be made in the definition of the linkage members so that the safety of the css in this function is more clear to users. If this is sufficient, I will make the change in a v2 patchset.
Sorry, I replied to this email earlier but it had some issues with plain text. Please ignore the first reply of mine (the one with HTML). I'm resending the email below. Thank you Johannes, Roman, and Yosry for reviewing this patch! On Thu, Jul 25, 2024 at 3:34 PM Yosry Ahmed <yosryahmed@google.com> wrote: > On Thu, Jul 25, 2024 at 1:43 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > What does this buy us? The tryget is cheap. > > mem_cgroup_iter() is not an easy function to follow, so I personally > appreciate the simplicity gains tbh. Yes, the main intention here was to simplify the code's readability. > This reads to me like it is intentional that RCU protection is enough > for @pos and @root, and that the sibling linkage is RCU protected by > design. Perhaps we could clarify this further (whether at > css_next_descendant_pre(), or above the definition of the linkage > members). Do we want to move forward with Yosry's suggestion to clarify that the sibling linkage is RCU-protected by design? Perhaps this clarification can be made in the definition of the linkage members so that the safety of the css in this function is more clear to users. If this is sufficient, I will make the change in a v2 patchset.
On Thu, Aug 01, 2024 at 03:32:53PM -0700, Kinsey Ho wrote: > Sorry, I replied to this email earlier but it had some issues with plain > text. Please ignore the first reply of mine (the one with HTML). I'm resending > the email below. > > Thank you Johannes, Roman, and Yosry for reviewing this patch! > > On Thu, Jul 25, 2024 at 3:34 PM Yosry Ahmed <yosryahmed@google.com> wrote: > > On Thu, Jul 25, 2024 at 1:43 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > > What does this buy us? The tryget is cheap. > > > > mem_cgroup_iter() is not an easy function to follow, so I personally > > appreciate the simplicity gains tbh. > > Yes, the main intention here was to simplify the code's readability. > > > This reads to me like it is intentional that RCU protection is enough > > for @pos and @root, and that the sibling linkage is RCU protected by > > design. Perhaps we could clarify this further (whether at > > css_next_descendant_pre(), or above the definition of the linkage > > members). > > Do we want to move forward with Yosry's suggestion to clarify that the > sibling linkage is RCU-protected by design? Perhaps this clarification > can be made in the definition of the linkage members so that the > safety of the css in this function is more clear to users. If this is > sufficient, I will make the change in a v2 patchset. Yes, that sounds like a good way forward to me.
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index 7e2eb091049a..4cbab85e2e56 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -75,7 +75,7 @@ struct lruvec_stats_percpu; struct lruvec_stats; struct mem_cgroup_reclaim_iter { - struct mem_cgroup *position; + struct mem_cgroup __rcu *position; /* scan generation, increased every round-trip */ unsigned int generation; }; diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 960371788687..062bfeee799c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1019,20 +1019,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, else if (reclaim->generation != iter->generation) goto out_unlock; - while (1) { - pos = READ_ONCE(iter->position); - if (!pos || css_tryget(&pos->css)) - break; - /* - * css reference reached zero, so iter->position will - * be cleared by ->css_released. However, we should not - * rely on this happening soon, because ->css_released - * is called from a work queue, and by busy-waiting we - * might block it. So we clear iter->position right - * away. - */ - (void)cmpxchg(&iter->position, pos, NULL); - } + pos = rcu_dereference(iter->position); } else if (prev) { pos = prev; } @@ -1073,9 +1060,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, */ (void)cmpxchg(&iter->position, pos, memcg); - if (pos) - css_put(&pos->css); - if (!memcg) iter->generation++; }
To obtain the pointer to the saved memcg position, mem_cgroup_iter() currently holds css->refcnt during memcg traversal only to put css->refcnt at the end of the routine. This isn't necessary as an rcu_read_lock is already held throughout the function. Remove css->refcnt usage during traversal by leveraging RCU. Signed-off-by: Kinsey Ho <kinseyho@google.com> --- include/linux/memcontrol.h | 2 +- mm/memcontrol.c | 18 +----------------- 2 files changed, 2 insertions(+), 18 deletions(-)