memcg, oom: move out_of_memory back to the charge path

Commit 3812c8c8f395 ("mm: memcg: do not trap chargers with full
callstack on OOM") has changed the ENOMEM semantic of memcg charges.
Rather than invoking the oom killer from the charging context it delays
the oom killer to the page fault path (pagefault_out_of_memory).  This
in turn means that many users (e.g.  slab or g-u-p) will get ENOMEM when
the corresponding memcg hits the hard limit and the memcg is is OOM.
This is behavior is inconsistent with !memcg case where the oom killer
is invoked from the allocation context and the allocator keeps retrying
until it succeeds.

The difference in the behavior is user visible.  mmap(MAP_POPULATE)
might result in not fully populated ranges while the mmap return code
doesn't tell that to the userspace.  Random syscalls might fail with
ENOMEM etc.

The primary motivation of the different memcg oom semantic was the
deadlock avoidance.  Things have changed since then, though.  We have an
async oom teardown by the oom reaper now and so we do not have to rely
on the victim to tear down its memory anymore.  Therefore we can return
to the original semantic as long as the memcg oom killer is not handed
over to the users space.

There is still one thing to be careful about here though.  If the oom
killer is not able to make any forward progress - e.g.  because there is
no eligible task to kill - then we have to bail out of the charge path
to prevent from same class of deadlocks.  We have basically two options
here.  Either we fail the charge with ENOMEM or force the charge and
allow overcharge.  The first option has been considered more harmful
than useful because rare inconsistencies in the ENOMEM behavior is hard
to test for and error prone.  Basically the same reason why the page
allocator doesn't fail allocations under such conditions.  The later
might allow runaways but those should be really unlikely unless somebody
misconfigures the system.  E.g.  allowing to migrate tasks away from the
memcg to a different unlimited memcg with move_charge_at_immigrate
disabled.

Link: http://lkml.kernel.org/r/20180628151101.25307-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 9 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 50e3e807b427..57a202f31683 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -507,16 +507,16 @@ unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
 void mem_cgroup_print_oom_info(struct mem_cgroup *memcg,
 				struct task_struct *p);
 
-static inline void mem_cgroup_oom_enable(void)
+static inline void mem_cgroup_enter_user_fault(void)
 {
-	WARN_ON(current->memcg_may_oom);
-	current->memcg_may_oom = 1;
+	WARN_ON(current->in_user_fault);
+	current->in_user_fault = 1;
 }
 
-static inline void mem_cgroup_oom_disable(void)
+static inline void mem_cgroup_exit_user_fault(void)
 {
-	WARN_ON(!current->memcg_may_oom);
-	current->memcg_may_oom = 0;
+	WARN_ON(!current->in_user_fault);
+	current->in_user_fault = 0;
 }
 
 static inline bool task_in_memcg_oom(struct task_struct *p)
@@ -961,11 +961,11 @@ static inline void mem_cgroup_handle_over_high(void)
 {
 }
 
-static inline void mem_cgroup_oom_enable(void)
+static inline void mem_cgroup_enter_user_fault(void)
 {
 }
 
-static inline void mem_cgroup_oom_disable(void)
+static inline void mem_cgroup_exit_user_fault(void)
 {
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1827f4a7a6de..066a2c328653 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -722,7 +722,7 @@ struct task_struct {
 	unsigned			restore_sigmask:1;
 #endif
 #ifdef CONFIG_MEMCG
-	unsigned			memcg_may_oom:1;
+	unsigned			in_user_fault:1;
 #ifndef CONFIG_SLOB
 	unsigned			memcg_kmem_skip_account:1;
 #endif