Re: [RFC Patch 0/2] mm: Add parameters to make kernel behavior at memory error on dirty cache selectable

[Simon Jeons <simon.jeons@gmail.com>]

On 04/11/2013 09:49 PM, Andi Kleen wrote:
>> As a result, if the dirty cache includes user data, the data is lost,
>> and data corruption occurs if an application uses old data.

Hi Andi,

Could you give me the link of your mce testcase?

> The application cannot use old data, the kernel code kills it if it
> would do that. And if it's IO data there is an EIO triggered.
>
> iirc the only concern in the past was that the application may miss
> the asynchronous EIO because it's cleared on any fd access.
>
> This is a general problem not specific to memory error handling,
> as these asynchronous IO errors can happen due to other reason
> (bad disk etc.)
>
> If you're really concerned about this case I think the solution
> is to make the EIO more sticky so that there is a higher chance
> than it gets returned.  This will make your data much more safe,
> as it will cover all kinds of IO errors, not just the obscure memory
> errors.
>
> Or maybe have a panic knob on any IO error for any case if you don't
> trust your application to check IO syscalls. But I would rather
> have better EIO reporting than just giving up like this.
>
> The problem of tying it just to any dirty data for memory errors
> is that most anonymous data is dirty and it doesn't have this problem
> at all (because the signals handle this and they cannot be lost)
>
> And that is a far more common case than this relatively unlikely
> case of dirty IO data.
>
> So just doing it for "dirty" is not the right knob.
>
> Basically I'm saying if you worry about unreliable IO error reporting
> fix IO error reporting, don't add random unnecessary panics to
> the memory error handling.
>
> BTW my suspicion is that if you approach this from a data driven
> perspective: that is measure how much such dirty data is typically
> around in comparison to other data it will be unlikely. Such
> a study can be done with the "page-types" program in tools/vm
>
> -Andi
>
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