Date:	Tue, 7 Apr 2009 14:13:00 -0500
From:	Robin Holt <holt@....com>
To:	Andi Kleen <andi@...stfloor.org>, Russ Anderson <rja@....com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH] [0/16] POISON: Intro

How does this overlap with the bad page quarantine that ia64 uses
following an MCA?

Robin

On Tue, Apr 07, 2009 at 05:09:56PM +0200, Andi Kleen wrote:
> 
> Upcoming Intel CPUs have support for recovering from some memory errors. This
> requires the OS to declare a page "poisoned", kill the processes associated
> with it and avoid using it in the future. This patchkit implements
> the necessary infrastructure in the VM.
> 
> To quote the overview comment:
> 
>  * High level machine check handler. Handles pages reported by the
>  * hardware as being corrupted usually due to a 2bit ECC memory or cache
>  * failure.
>  *
>  * This focusses on pages detected as corrupted in the background.
>  * When the current CPU tries to consume corruption the currently
>  * running process can just be killed directly instead. This implies
>  * that if the error cannot be handled for some reason it's safe to
>  * just ignore it because no corruption has been consumed yet. Instead
>  * when that happens another machine check will happen.
>  *
>  * Handles page cache pages in various states. The tricky part
>  * here is that we can access any page asynchronous to other VM
>  * users, because memory failures could happen anytime and anywhere,
>  * possibly violating some of their assumptions. This is why this code
>  * has to be extremely careful. Generally it tries to use normal locking
>  * rules, as in get the standard locks, even if that means the
>  * error handling takes potentially a long time.
>  *
>  * Some of the operations here are somewhat inefficient and have non
>  * linear algorithmic complexity, because the data structures have not
>  * been optimized for this case. This is in particular the case
>  * for the mapping from a vma to a process. Since this case is expected
>  * to be rare we hope we can get away with this.
> 
> The code consists of a the high level handler in mm/memory-failure.c, 
> a new page poison bit and various checks in the VM to handle poisoned
> pages.
> 
> The main target right now is KVM guests, but it works for all kinds
> of applications.
> 
> For the KVM use there was need for a new signal type so that
> KVM can inject the machine check into the guest with the proper
> address. This in theory allows other applications to handle
> memory failures too. The expection is that near all applications
> won't do that, but some very specialized ones might. 
> 
> This is not fully complete yet, in particular there are still ways
> to access poison through various ways (crash dump, /proc/kcore etc.)
> that need to be plugged too.
> 
> Also undoubtedly the high level handler still has bugs and cases
> it cannot recover from. For example nonlinear mappings deadlock right now
> and a few other cases lose references. Huge pages are not supported
> yet. Any additional testing, reviewing etc. welcome. 
> 
> The patch series requires the earlier x86 MCE feature series for the x86
> specific action optional part. The code can be tested without the x86 specific
> part using the injector, this only requires to enable the Kconfig entry
> manually in some Kconfig file (by default it is implicitely enabled
> by the architecture)
> 
> -Andi
> 
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