[PATCH kernel 06/15] powerpc/mm/iommu: Put pages on process exit

[Nicholas Piggin]

On Wed,  3 Aug 2016 18:40:47 +1000
Alexey Kardashevskiy <aik at ozlabs.ru> wrote:

> At the moment VFIO IOMMU SPAPR v2 driver pins all guest RAM pages when
> the userspace starts using VFIO. When the userspace process finishes,
> all the pinned pages need to be put; this is done as a part of
> the userspace memory context (MM) destruction which happens on
> the very last mmdrop().
> 
> This approach has a problem that a MM of the userspace process
> may live longer than the userspace process itself as kernel threads
> use userspace process MMs which was runnning on a CPU where
> the kernel thread was scheduled to. If this happened, the MM remains
> referenced until this exact kernel thread wakes up again
> and releases the very last reference to the MM, on an idle system this
> can take even hours.
> 
> This references and caches MM once per container and adds tracking
> how many times each preregistered area was registered in
> a specific container. This way we do not depend on @current pointing to
> a valid task descriptor.
> 
> This changes the userspace interface to return EBUSY if memory is
> already registered (mm_iommu_get() used to increment the counter);
> however it should not have any practical effect as the only
> userspace tool available now does register memory area once per
> container anyway.
> 
> As tce_iommu_register_pages/tce_iommu_unregister_pages are called
> under container->lock, this does not need additional locking.
> 
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>

Reviewed-by: Nicholas Piggin <npiggin at gmail.com>