Re: error at compaction (Re: mmotm 2010-04-15-14-42 uploaded

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 20 Apr 2010 12:58:43 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Tue, Apr 20, 2010 at 12:07 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Tue, 20 Apr 2010 11:39:46 +0900
> >> Dumb question. Why can't we call arch_alloc_page and kernel_map_pages
> >> as interrupt disabled? It's deadlock issue or latency issue?
> >> I don't found any comment about it.
> >> It should have added the comment around that functions. :)
> >>
> >
> > I guess it's from the same reason as vfree(), which can't be called under
> > irq-disabled.
> >
> > Both of them has to flush TLB of all cpus. At flushing TLB (of other cpus), cpus has
> > to send IPI via smp_call_function. What I know from old stories is below.
> >
> > At sendinf IPI, usual sequence is following. (This may be old.)
> >
> >        spin_lock(&ipi_lock);
> >                set up cpu mask for getting notification from other cpu for declearing
> >                "I received IPI and finished my own work".
> >        spin_unlock(&ipi_lock);
> >
> > Then,
> >          CPU0                             CPU1
> >
> >    irq_disable (somewhere)             spin_lock
> >                                        send IPI and wait for notification.
> >    spin_lock()
> >
> > deadlock.  Seeing decription of kernel/smp.c::smp_call_function_many(), it says
> > this function should not be called under irq-disabled.
> > (Maybe the same kind of spin-wait deadlock can happen.)
> >
> 
> Thanks for kind explanation.
> Actually I guessed TLB issue but I can't find any glue point which
> connect tlb flush to smp_call_function_xxx. :(
> 
> Now look at the __native_flush_tlb_global.
> It just read and write cr4 with just mask off X86_CR4_PGE.
> So i don't know how connect this and smp_schedule_xxxx.
> Hmm,, maybe APIC?
> 
> Sorry for dumb question.
> 
Hmm...seeing again,

arch/x86/mm/pageattr.c::kernel_map_pages() says:

1293         /*
1294          * We should perform an IPI and flush all tlbs,
1295          * but that can deadlock->flush only current cpu:
1296          */

Wow. It just flush only local cpu. Then, no IPI.

Hmm...all other archs does the same thing ? If so, kernel_map_pages()
can be called under irq_disabled. The author of kernel_map_pages() 
is aware that this can be called under irq-disabled.

Hmm...

Thanks,
-Kame