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

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Message-ID: <20130411181004.GK16732@two.firstfloor.org>
Date:	Thu, 11 Apr 2013 20:10:04 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Mitsuhiro Tanino <mitsuhiro.tanino.gm@...achi.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC Patch 0/2] mm: Add parameters to make kernel behavior at
 memory error on dirty cache selectable

On Thu, Apr 11, 2013 at 11:23:08AM -0400, Naoya Horiguchi wrote:
> On Thu, Apr 11, 2013 at 03:49:16PM +0200, 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.
> > 
> > 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.
> 
> I'm interested in this topic, and in previous discussion, what I was said
> is that we can't expect user applications to change their behaviors when
> they get EIO, so globally changing EIO's stickiness is not a great approach.

Not sure. Some of the current behavior may be dubious and it may 
be possible to change it. But would need more analysis.

I don't think we're concerned that much about "correct" applications,
but more ones that do not check everything. So returning more
errors should be safer.

For example you could have a sysctl that enables always stick
IO error -- that keeps erroring until it is closed.

> I'm working on a new pagecache tag based mechanism to solve this.
> But it needs time and more discussions.
> So I guess Tanino-san suggests giving up on dirty pagecache errors
> as a quick solution.

A quick solution would be enabling panic for any asynchronous IO error.
I don't think the memory error code is the right point to hook into.

-Andi
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