Date:	Sat, 13 Jun 2009 00:14:31 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andi Kleen <andi@...stfloor.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/5] HWPOISON: define VM_FAULT_HWPOISON to 0 when
	feature is disabled

On Fri, Jun 12, 2009 at 11:36:20PM +0800, Ingo Molnar wrote:
> 
> * Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > On Fri, Jun 12, 2009 at 09:17:54PM +0800, Ingo Molnar wrote:
> > > 
> > > * Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > 
> > > > Hi Ingo,
> > > > 
> > > > On Fri, Jun 12, 2009 at 07:22:58PM +0800, Ingo Molnar wrote:
> > > > > 
> > > > > * Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > > > 
> > > > > > So as to eliminate one #ifdef in the c source.
> > > > > > 
> > > > > > Proposed by Nick Piggin.
> > > > > > 
> > > > > > CC: Nick Piggin <npiggin@...e.de>
> > > > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > > > > ---
> > > > > >  arch/x86/mm/fault.c |    3 +--
> > > > > >  include/linux/mm.h  |    7 ++++++-
> > > > > >  2 files changed, 7 insertions(+), 3 deletions(-)
> > > > > > 
> > > > > > --- sound-2.6.orig/arch/x86/mm/fault.c
> > > > > > +++ sound-2.6/arch/x86/mm/fault.c
> > > > > > @@ -819,14 +819,13 @@ do_sigbus(struct pt_regs *regs, unsigned
> > > > > >  	tsk->thread.error_code	= error_code;
> > > > > >  	tsk->thread.trap_no	= 14;
> > > > > >  
> > > > > > -#ifdef CONFIG_MEMORY_FAILURE
> > > > > >  	if (fault & VM_FAULT_HWPOISON) {
> > > > > >  		printk(KERN_ERR
> > > > > >  	"MCE: Killing %s:%d due to hardware memory corruption fault at %lx\n",
> > > > > >  			tsk->comm, tsk->pid, address);
> > > > > >  		code = BUS_MCEERR_AR;
> > > > > >  	}
> > > > > > -#endif
> > > > > 
> > > > > Btw., anything like this should happen in close cooperation with 
> > > > > the x86 tree, not as some pure MM feature. I dont see Cc:s and 
> > > > > nothing that indicates that realization. What's going on here?
> > > > 
> > > > Ah sorry for the ignorance!  Andi has a nice overview of the big 
> > > > picture here: http://lkml.org/lkml/2009/6/3/371
> > > > 
> > > > In the above chunk, the process is trying to access the already 
> > > > corrupted page and thus shall be killed, otherwise it will either 
> > > > silently consume corrupted data, or will trigger another (deadly) 
> > > > MCE event and bring down the whole machine.
> > > 
> > > This seems like trying to handle a failure mode that cannot be and 
> > > shouldnt be 'handled' really. If there's an 'already corrupted' page 
> > > then the box should go down hard and fast, and we should not risk 
> > > _even more user data corruption_ by trying to 'continue' in the hope 
> > > of having hit some 'harmless' user process that can be killed ...
> > > 
> > > So i find the whole feature rather dubious - what's the point? We 
> > > should panic at this point - we just corrupted user data so that 
> > > piece of hardware cannot be trusted. Nor can any subsequent kernel 
> > > bug messages be trusted.
> > > 
> > > Do we really want this in the core Linux VM and in the architecture 
> > > pagefault handling code and elsewhere? Am i the only one who finds 
> > > this concept of 'handling' user data corruption rather dubious?
> > 
> > - The corrupted data only impacts one or more process(es)
> > - The corrupted data has not be consumed yet
> > 
> > The data corruption has not caused real hurt yet, and can be 
> > isolated to prevent future accesses.  So it makes sense to just 
> > kill the impacted process(es).
> 
> Dunno, this just looks like a license to allow more crappy hardware, 
> hm? I'm all for _logging_ errors, but hwpoison is not about that: it 
> is about allowing the hardware to limp along in 'enterprise' setups, 
> with a (false looking) 'guarantee' that everything is fine.
> 
> There's no guarantee that the fault doesnt hit something critical - 
> and by allowing 'harmless' faults we push up the noise level.
> 
> Any move from us to make faulty hardware more acceptable by 
> "handling" it in a percentage of cases (and crashing/corrupting in 
> other cases) is futile IMHO - it just sends the wrong general 
> message.
> 
> I.e. i think this thinking misses the general harm on for example 
> the quality of kernel bugreports: if such a system corrupts memory, 
> and crashes in a weird way - we'll get a weird kernel-crash report. 
> If it 'only' corrupts some user process in a 'harmless' way, we wont 
> get a crash report. Say the kernel crashes in 10% of the cases, 
> user-space crashes in 90% of the cases.
> 
> If we allow that 90% to continue, we make the 10% "bad" crash 
> proportion more prominent in our stats too. I.e. by allowing 
> 'harmless' bugs to be more acceptable in practice, we indirectly 
> increase the proportion of _bad_ crashes as well.
> 
> Do you accept that general point or am i wrong?
> 
> Computing along the von Neumann principles really depends on having 
> a sufficiently well working piece of hardware that one can trust 
> with a reasonable certainty. Probabilistic computing is fine too in 
> certain isolated fields where you say want some probabilistic result 
> to begin with (say the result of some property of the physical 
> world) - but in general purpose hardware i doubt it's the right kind 
> of approach ...

NAND flash is crappy - it is continuously rotting - it's wrong to
encourage its usage by inventing wear leveling and checksum algorithms
and to make SSD on top of them.

wireless network is crappy - it so much more unreliable than fibre networks.

PC servers are crappy - google invented the google file system? Damn it!


HWPOISON is a reliability enabling feature - if it enables prevalent
of crappy hardwares, let's celebrate changing the world~~

Thanks,
Fengguang

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