On Mon, Jun 01, 2015 at 01:14:52PM -0500, Clark Williams wrote: > On Fri, 29 May 2015 14:26:14 -0700 > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Fri, 29 May 2015 10:48:15 -0500 Clark Williams <williams@xxxxxxxxxx> wrote: > > > > > The irqs_disabled() check in mem_cgroup_swapout() fails on the latest > > > RT kernel because RT mutexes do not disable interrupts when held. Change > > > the test for the lock being held to use spin_is_locked. > > > > > > ... > > > > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -5845,7 +5845,7 @@ void mem_cgroup_swapout(struct page *page, > > > swp_entry_t entry) page_counter_uncharge(&memcg->memory, 1); > > > > > > /* XXX: caller holds IRQ-safe mapping->tree_lock */ > > > - VM_BUG_ON(!irqs_disabled()); > > > + VM_BUG_ON(!spin_is_locked(&page_mapping(page)->tree_lock)); > > > > > > mem_cgroup_charge_statistics(memcg, page, -1); > > > memcg_check_events(memcg, page); > > > > spin_is_locked() returns zero on uniprocessor builds. The results will > > be unhappy. > > > > I suggest just deleting the check. > > Guess this is Johannes call. We can just #ifdef it out and that would > remain the same when we finally merge PREEMPT_RT in mainline. > > If Johannes wants to keep the check on non-RT, here's a patch: Andrew's suggestion makes sense, we can probably just delete the check as long as we keep the comment. That being said, I think it's a little weird that this doesn't work: spin_lock_irq() BUG_ON(!irqs_disabled()) spin_unlock_irq() I'd expect that if you change the meaning of spin_lock_irq() from "mask hardware interrupts" to "disable preemption by tophalf", you would update the irqs_disabled() macro to match. Most people using this check probably don't care about the hardware state, only that they don't get preempted by an interfering interrupt handler, no? --- From: Johannes Weiner <hannes@xxxxxxxxxxx> Date: Mon, 1 Jun 2015 14:30:49 -0400 Subject: [PATCH] mm: memcontrol: fix false-positive VM_BUG_ON() on -rt On -rt, the VM_BUG_ON(!irqs_disabled()) triggers inside the memcg swapout path because the spin_lock_irq(&mapping->tree_lock) in the caller doesn't actually disable the hardware interrupts - which is fine, because on -rt the tophalves run in process context and so we are still safe from preemption while updating the statistics. Remove the VM_BUG_ON() but keep the comment of what we rely on. Reported-by: Clark Williams <williams@xxxxxxxxxx> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> --- mm/memcontrol.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 14c2f20..977f7cd 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5833,9 +5833,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry) if (!mem_cgroup_is_root(memcg)) page_counter_uncharge(&memcg->memory, 1); - /* XXX: caller holds IRQ-safe mapping->tree_lock */ - VM_BUG_ON(!irqs_disabled()); - + /* Caller disabled preemption with mapping->tree_lock */ mem_cgroup_charge_statistics(memcg, page, -1); memcg_check_events(memcg, page); } -- 2.4.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>