[patch] mm: memcontrol: uncharge pages on swapout fix

[Johannes Weiner]

Vladimir notes:

> > +   local_irq_disable();
> > +   mem_cgroup_charge_statistics(memcg, page, -1);
> > +   memcg_check_events(memcg, page);
> > +   local_irq_enable();
>
> AFAICT mem_cgroup_swapout() is called under mapping->tree_lock with irqs
> disabled, so we should use irq_save/restore here.

But this function doesn't actually need to be called under the tree
lock.  So for now, simply remove the irq-disabling altogether and rely
on the caller's IRQ state.  Later on, we'll move it out from there and
add back the simple, non-saving IRQ-disabling.

Reported-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8dc46aa9ae8f..c688fb73ff35 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5806,6 +5806,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 	VM_BUG_ON_PAGE(page_count(page), page);
 
+	/* XXX: caller holds IRQ-safe mapping->tree_lock */
+	VM_BUG_ON(!irqs_disabled());
+
 	if (!do_swap_account)
 		return;
 
@@ -5827,10 +5830,8 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	if (!mem_cgroup_is_root(memcg))
 		page_counter_uncharge(&memcg->memory, 1);
 
-	local_irq_disable();
 	mem_cgroup_charge_statistics(memcg, page, -1);
 	memcg_check_events(memcg, page);
-	local_irq_enable();
 }
 
 /**
-- 
2.1.2
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