Linux Virtualization - Aug 2007 - [PATCH] Fix lazy mode vmalloc synchronization for paravirt

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.

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.

Currently for 2.6.23, but we'll want to backport to -stable as well.

Zach
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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

Jeremy Fitzhardinge wrote:
> No, under Xen the kernel/hypervisor PMD is not shared between processes,
> so this is still used when PAE is enabled.
>   
Ahh, yes.  So this was a lucky catch for us.  Non-PAE kernels seem to be 
increasing in value at antique sales.

Zach