Re: Interaction between Xen and XFS: stray RW mappings

[Jeremy Fitzhardinge <jeremy@goop.org>]

Andi Kleen wrote:
> Jeremy Fitzhardinge <jeremy@goop.org> writes:
>   
>> Yes, that's precisely the problem.  xfs does delay the unmap, leaving
>> stray mappings, which upsets Xen.
>>     
>
> Again it not just upsets Xen, keeping mappings to freed pages is wrong generally 
> and violates the x86 (and likely others like PPC) architecture because it can 
> cause illegal caching attribute aliases.
>
> The patch that went into the tree was really not correct -- this
> bogus optimization should have been unconditionally removed
> or if you really wanted an ifdef made dependent on !CONFIG_XEN &&
> !CONFIG_AGP (and likely && !CONFIG_DRM && !CONFIG_anything else that
> uses uncached mappings in memory). 
>
> You just worked around the obvious failure and leave the non obvious
> rare corruptions in, which isn't a good strategy.

Well, at least it becomes a known issue and/or placeholder for when Nick
does his grand unified vmap manager.  I guess a clean workaround would
be to add a CONFIG_XFS_LAZY_UNMAP, and do it at the Kconfig level... 
I'll cook up a patch.

    J