mm: memcontrol: bring back the VM_BUG_ON() in mem_cgroup_swapout()
Clark stumbled over a VM_BUG_ON() in -RT which was then was removed by
Johannes in commit f371763a79d ("mm: memcontrol: fix false-positive
VM_BUG_ON() on -rt"). The comment before that patch was a tiny bit better
than it is now. While the patch claimed to fix a false-postive on -RT
this was not the case. None of the -RT folks ACKed it and it was not a
false positive report. That was a *real* problem.
This patch updates the comment that is improper because it refers to
"disabled preemption" as a consequence of that lock being taken. A
spin_lock() disables preemption, true, but in this case the code relies on
the fact that the lock _also_ disables interrupts once it is acquired.
And this is the important detail (which was checked the VM_BUG_ON()) which
needs to be pointed out. This is the hint one needs while looking at the
code. It was explained by Johannes on the list that the per-CPU variables
are protected by local_irq_save(). The BUG_ON() was helpful. This code
has been workarounded in -RT in the meantime. I wouldn't mind running
into more of those if the code in question uses *special* kind of locking
since now there is no verification (in terms of lockdep or BUG_ON()) and
therefore I bring the VM_BUG_ON() check back in.
The two functions after the comment could also have a "local_irq_save()"
dance around them in order to serialize access to the per-CPU variables.
This has been avoided because the interrupts should be off.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Clark Williams <williams@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index acb93c5..1af0575 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5965,7 +5965,13 @@
if (!mem_cgroup_is_root(memcg))
page_counter_uncharge(&memcg->memory, 1);
- /* Caller disabled preemption with mapping->tree_lock */
+ /*
+ * Interrupts should be disabled here because the caller holds the
+ * mapping->tree_lock lock which is taken with interrupts-off. It is
+ * important here to have the interrupts disabled because it is the
+ * only synchronisation we have for udpating the per-CPU variables.
+ */
+ VM_BUG_ON(!irqs_disabled());
mem_cgroup_charge_statistics(memcg, page, -1);
memcg_check_events(memcg, page);
}