linux-kernel - Re: mpol_to

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Date:	Mon, 15 Oct 2012 23:10:09 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	bhutchings@...arflare.com, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mpol_to_str revisited.

On Tue, 16 Oct 2012, KOSAKI Motohiro wrote:

> >> I don't think 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a is right fix.
> >
> > It's certainly not a complete fix, but I think it's a much better result
> > of the race, i.e. we don't panic anymore, we simply fail the read()
> > instead.
> 
> Even though 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a itself is simple. It bring
> to caller complex. That's not good and have no worth.
> 

Before: the kernel panics, all workloads cease.
After: the file shows garbage, all workloads continue.

This is better, in my opinion, but at best it's only a judgment call and 
has no effect on anything.

I agree it would be better to respect the return value of mpol_to_str() 
since there are other possible error conditions other than a freed 
mempolicy, but let's not consider reverting 80de7c3138.  It is obviously 
not a full solution to the problem, though, and we need to serialize with 
task_lock().

Dave, are you interested in coming up with a patch?
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