linux-kernel - Re: [PATCH] Fix lazy mode vmalloc synchronization for paravirt

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Date:	Tue, 21 Aug 2007 23:28:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	Ben Collins <ben.collins@...ntu.com>
Subject: Re: [PATCH] Fix lazy mode vmalloc synchronization for paravirt

Zachary Amsden wrote:
> Found this looping Ubuntu installs with VMI.
>
> If unlucky enough to hit a vmalloc sync fault during a lazy mode
> operation (from an IRQ handler for a module which was not yet
> populated in current page directory, or from inside copy_one_pte,
> which touches swap_map, and hit in an unused 4M region), the required
> PDE update would never get flushed, causing an infinite page fault loop.

Ah, good catch.

> This bug affects any paravirt-ops backend which uses lazy updates, I
> believe that makes it a bug in Xen, VMI and lguest.  It only happens
> on LOWMEM kernels. 

No, under Xen the kernel/hypervisor PMD is not shared between processes,
so this is still used when PAE is enabled.

    J
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