mm: memcg: handle non-error OOM situations more gracefully

Commit 3812c8c8f395 ("mm: memcg: do not trap chargers with full
callstack on OOM") assumed that only a few places that can trigger a
memcg OOM situation do not return VM_FAULT_OOM, like optional page cache
readahead.  But there are many more and it's impractical to annotate
them all.

First of all, we don't want to invoke the OOM killer when the failed
allocation is gracefully handled, so defer the actual kill to the end of
the fault handling as well.  This simplifies the code quite a bit for
added bonus.

Second, since a failed allocation might not be the abrupt end of the
fault, the memcg OOM handler needs to be re-entrant until the fault
finishes for subsequent allocation attempts.  If an allocation is
attempted after the task already OOMed, allow it to bypass the limit so
that it can quickly finish the fault and invoke the OOM killer.

Reported-by: azurIt <azurit@pobox.sk>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 6 deletions

diff --git a/mm/memory.c b/mm/memory.c
index f7b7692c05ed..1311f26497e6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3865,15 +3865,21 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * space.  Kernel faults are handled more gracefully.
 	 */
 	if (flags & FAULT_FLAG_USER)
-		mem_cgroup_enable_oom();
+		mem_cgroup_oom_enable();
 
 	ret = __handle_mm_fault(mm, vma, address, flags);
 
-	if (flags & FAULT_FLAG_USER)
-		mem_cgroup_disable_oom();
-
-	if (WARN_ON(task_in_memcg_oom(current) && !(ret & VM_FAULT_OOM)))
-		mem_cgroup_oom_synchronize();
+	if (flags & FAULT_FLAG_USER) {
+		mem_cgroup_oom_disable();
+                /*
+                 * The task may have entered a memcg OOM situation but
+                 * if the allocation error was handled gracefully (no
+                 * VM_FAULT_OOM), there is no need to kill anything.
+                 * Just clean up the OOM state peacefully.
+                 */
+                if (task_in_memcg_oom(current) && !(ret & VM_FAULT_OOM))
+                        mem_cgroup_oom_synchronize(false);
+	}
 
 	return ret;
 }