Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limitinginfrastructure

[KAMEZAWA Hiroyuki]

On Mon, 8 Mar 2010 10:44:47 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> > +/*
> > + * mem_cgroup_update_page_stat_locked() - update memcg file cache's accounting
> > + * @page:	the page involved in a file cache operation.
> > + * @idx:	the particular file cache statistic.
> > + * @charge:	true to increment, false to decrement the statistic specified
> > + *		by @idx.
> > + *
> > + * Update memory cgroup file cache's accounting from a locked context.
> > + *
> > + * NOTE: must be called with mapping->tree_lock held.
> > + */
> > +void mem_cgroup_update_page_stat_locked(struct page *page,
> > +			enum mem_cgroup_write_page_stat_item idx, bool charge)
> > +{
> > +	struct address_space *mapping = page_mapping(page);
> > +	struct page_cgroup *pc;
> > +
> > +	if (mem_cgroup_disabled())
> > +		return;
> > +	WARN_ON_ONCE(!irqs_disabled());
> > +	WARN_ON_ONCE(mapping && !spin_is_locked(&mapping->tree_lock));
> > +
> I think this is a wrong place to insert assertion.
> The problem about page cgroup lock is that it can be interrupted in current implementation.
> So,
> 
> a) it must not be aquired under another lock which can be aquired in interrupt context,
>    such as mapping->tree_lock, to avoid:
> 
> 		context1			context2
> 					lock_page_cgroup(pcA)
> 	spin_lock_irq(&tree_lock)
> 		lock_page_cgroup(pcA)		<interrupted>
> 		=>fail				spin_lock_irqsave(&tree_lock)
> 						=>fail
> 
> b) it must not be aquired in interrupt context to avoid:
> 
> 	lock_page_cgroup(pcA)
> 		<interrupted>
> 		lock_page_cgroup(pcA)
> 		=>fail
> 
> I think something like this would be better:
> 
> @@ -83,8 +83,14 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)
>         return page_zonenum(pc->page);
>  }
> 
> +#include <linux/irqflags.h>
> +#include <linux/hardirq.h>
>  static inline void lock_page_cgroup(struct page_cgroup *pc)
>  {
> +#ifdef CONFIG_DEBUG_VM
> +       WARN_ON_ONCE(irqs_disabled());
> +       WARN_ON_ONCE(in_interrupt());
> +#endif
>         bit_spin_lock(PCG_LOCK, &pc->flags);
>  }
> 
> > +	pc = lookup_page_cgroup(page);
> > +	if (unlikely(!pc) || !PageCgroupUsed(pc))
> > +		return;
> > +	mem_cgroup_update_page_stat(pc, idx, charge);
> > +}
> > +EXPORT_SYMBOL_GPL(mem_cgroup_update_page_stat_locked);
> > +
> > +/*
> > + * mem_cgroup_update_page_stat_unlocked() - update memcg file cache's accounting
> > + * @page:	the page involved in a file cache operation.
> > + * @idx:	the particular file cache statistic.
> > + * @charge:	true to increment, false to decrement the statistic specified
> > + *		by @idx.
> > + *
> > + * Update memory cgroup file cache's accounting from an unlocked context.
> > + */
> > +void mem_cgroup_update_page_stat_unlocked(struct page *page,
> > +			enum mem_cgroup_write_page_stat_item idx, bool charge)
> > +{
> > +	struct page_cgroup *pc;
> > +
> > +	if (mem_cgroup_disabled())
> > +		return;
> > +	pc = lookup_page_cgroup(page);
> > +	if (unlikely(!pc) || !PageCgroupUsed(pc))
> > +		return;
> > +	lock_page_cgroup(pc);
> > +	mem_cgroup_update_page_stat(pc, idx, charge);
> >  	unlock_page_cgroup(pc);
> >  }
> > +EXPORT_SYMBOL_GPL(mem_cgroup_update_page_stat_unlocked);
> >  
> IIUC, test_clear_page_writeback(at least) can be called under interrupt context.
> This means lock_page_cgroup() is called under interrupt context, that is,
> the case b) above can happen.
> hmm... I don't have any good idea for now except disabling irq around page cgroup lock
> to avoid all of these mess things.
> 

Hmm...simply IRQ-off for all updates ?
But IIRC, clear_writeback is done under treelock.... No ?

Thanks,
-Kame

> 
> Thanks,
> Daisuke Nishimura.
> 

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