linux-kernel - Re: [PATCH 6/9] uprobes: flush cache after xol write

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Date:	Mon, 29 Oct 2012 11:05:22 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Rabin Vincent <rabin@....in>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 6/9] uprobes: flush cache after xol write

On Fri, Oct 26, 2012 at 06:39:51PM +0200, Oleg Nesterov wrote:
> On 10/26, Ananth N Mavinakayanahalli wrote:
> >
> > On Thu, Oct 25, 2012 at 04:58:39PM +0200, Oleg Nesterov wrote:
> > > On 10/16, Rabin Vincent wrote:
> > > >
> > > > >> --- a/kernel/events/uprobes.c
> > > > >> +++ b/kernel/events/uprobes.c
> > > > >> @@ -1246,6 +1246,7 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe, unsigned long slot
> > > > >>       offset = current->utask->xol_vaddr & ~PAGE_MASK;
> > > > >>       vaddr = kmap_atomic(area->page);
> > > > >>       arch_uprobe_xol_copy(&uprobe->arch, vaddr + offset);
> > > > >> +     flush_dcache_page(area->page);
> > > > >>       kunmap_atomic(vaddr);
> > > > >
> > > > > I agree... but why under kmap_atomic?
> > > >
> > > > No real reason; I'll move it to after the unmap.
> > >
> > > OK. I assume you will send v2.
> > >
> > > But this patch looks like a bugfix, flush_dcache_page() is not a nop
> > > on powerpc. So perhaps we should apply this fix right now?
> >
> > Starting Power5, all Power processers have coherent caches.
> >
> > > OTOH, I do not understand this stuff, everything is nop on x86. And
> > > when I look into Documentation/cachetlb.txt I am starting to think
> > > that may be this needs flush_icache_user_range instead?
> > >
> > > Rabin, Ananth could you clarify this?
> >
> > Yes. We need flush_icache_user_range(). Though for x86 its always been a
> > nop, one never knows if there is some Power4 or older machine out there
> > that is still being used. We are fine for Power5 and later.
> 
> This is bad...
> 
> flush_icache_user needs vma. perhaps just to check VM_EXEC...
> 
> So let me repeat to be sure I really understand, do you confirm that
> _in general_ flush_dcache_page() is not enough?

flush_dcache_page() on powerpc already checks for
CPU_FTR_COHERENT_ICACHE. So, yes, that is enough.

Ananth

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