Re: [RFC] mm: change irqs_disabled() test to spin_is_locked() in mem_cgroup_swapout

[Johannes Weiner <hannes@cmpxchg.org>]

Hi Clark,

On Fri, May 29, 2015 at 10:48:15AM -0500, Clark Williams wrote:
> @@ -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);

It's not about the lock, it's about preemption.  The charge statistics
use __this_cpu operations and they're updated from process context and
interrupt context both.

This function really should do a local_irq_save().  I only added the
VM_BUG_ON() to document that we know the caller is holding an IRQ-safe
lock and so we don't need to bother with another level of IRQ saving.

So how does this translate to RT?  I don't know.  But if switching to
explicit IRQ toggling would help you guys out we can do that.  It is
in the swapout path after all, the optimization isn't that important.