Re: [Xen-devel] Is: [PATCH] x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode. Was: Re: [PATCH] x86/paravirt: Partially revert "remove lazy mode in interrupts"

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Sep 30, 2011 at 11:59:46AM +0200, Stefan Bader wrote:
> On 26.09.2011 21:34, Konrad Rzeszutek Wilk wrote:
> > On Mon, Sep 26, 2011 at 09:22:21AM -0700, Jeremy Fitzhardinge wrote:
> >> On 09/26/2011 06:13 AM, Konrad Rzeszutek Wilk wrote:
> >>> which has git commit b8bcfe997e46150fedcc3f5b26b846400122fdd9.
> >>>
> >>> The unintended consequence of removing the flushing of MMU
> >>> updates when doing kmap_atomic (or kunmap_atomic) is that we can
> >>> hit a dereference bug when processing a "fork()" under a heavy loaded
> >>> machine. Specifically we can hit:
> >>
> >> The patch is all OK, but I wouldn't have headlined it as a "partial
> >> revert" - the important point is that the pte updates in k(un)map_atomic
> >> need to be synchronous, regardless of whether we're in lazy_mmu mode.
> >>
> >> The fact that b8bcfe997e4 introduced the problem is interesting to note,
> >> but only somewhat relevant to the analysis of what's being fixed here.
> > 
> > Good point. How about
> > 
> 
> Limiting the cc's for just asking about status...

CC-ed you on my query to Andrew. If nothing happens in the next couple of
days can you ping him too please?

> 
> >>From 09966678dd645b68a422c9bf0223b13e73387302 Mon Sep 17 00:00:00 2001
> > From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Date: Fri, 23 Sep 2011 17:02:29 -0400
> > Subject: [PATCH] x86/paravirt: PTE updates in k(un)map_atomic need to be 
> > synchronous, regardless of lazy_mmu mode.
> > 
> > This patch fixes an outstanding issue that has been reported since 2.6.37.
> > Under a heavy loaded machine processing "fork()" calls could keepover with:
> > 
> I wonder whether this may have some effect on older kernels too. According to
> git the patch that removed the lines that are added back happened in 2.6.31.
> Probably it is not the same symptom... I would tend to have it applied all the
> way back but its always better to get some authoritative answer (maybe helps 
> the
> maintainers of longterm, too).

I think so, but I've only gotten bug reports from 2.6.37 and on - so I am being
cautious.
> 
> Anyway, since this is a somewhat painful bug to users, do you happen to know 
> how
> far this is in reaching the upstream kernel?

Just need an Ack from either akpm, or x86 maintainers. The x86 maintainers are
busy with the kernel.org mishap so ... andrew is our guy.

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