Re: [patch] rt: res_counter fix, v2

KAMEZAWA Hiroyuki
2009-02-13T00:30:48Z

On Fri, 13 Feb 2009 08:56:40 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Thu, 12 Feb 2009 22:28:33 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
> > * Ingo Molnar <mingo@elte.hu> [2009-02-12 12:28:54]:
> >
> > >
> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > >
> > > > On Thu, 12 Feb 2009 11:21:13 +0100
> > > > Ingo Molnar <mingo@elte.hu> wrote:
> > > The question is, are these local IRQ flags manipulations really needed
> > > in this code, and if yes, why?
> >
> > We needed the local IRQ flags, since these counters are updated from
> > page fault context and from reclaim context with lru_lock held with
> > IRQ's disabled. I've been thinking about replacing the spin lock with
> > seq lock, but have not gotten to it yet.
> >
> Hmm ? I can't understand. Why we have to disable IRQ here again ?
> And,
> - try_to_unmap() is called in shrink_page_list(), there, no zone->lru_lock.
> - page fault path doesn't hold zone->lru_lock.
>
> My concern is only shmem. But I think it doesn't call charge() within lock, actually

Clarification :)
res_counter_charge() is called from
- page fault
=> under down_read(mmap_sem), lock_page() may be held. IRQ=ENABLED)
- add_to_page_cache
=> under lock_page(), mapping->tree_lock is *not* held, IRQ=DISABLED
- shmem
=> info->lock is held, we use __GFP_NOWAIT here. IRQ=ENABLED
- shmem
=> info->lock is *not* held with GFP_KERNEL here, IRQ=ENABLED.
- migration
=> under lock_page() and mmap_sem, IRQ=ENABLED

res_counter_uncharge() is called from
- page_remove_rmap()//(Only when ANON)
=> anon_vma->lock and pte_lock(),lock_page() can be held. IRQ=ENABLED?
- remove_from_page_cache()
=> lock_page() and mapping->tree_lock is held, IRQ=DISABLED.

Summary:
"Charge" is considered as heavy operation and the call path is placed where the
thread can sleep, AMAP.
"Uncharge" is considered as light operation and call path is under some # of
spinlocks.

Bye,
-Kame