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

[Balbir Singh]

On Thu, Jul 14, 2016 at 3:16 PM, Alexey Kardashevskiy <aik@xxxxxxxxx> 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
> usually execute on a MM of a userspace process 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 fixes the issue by moving mm_iommu_cleanup() (the helper which puts
> pages) from destroy_context() (called on the last mmdrop()) to
> the arch-specific arch_exit_mmap() hook (called on the last mmput()).
> mmdrop() decrements the mm->mm_count which is a total reference number;
> mmput() decrements the mm->mm_users which is a number of user spaces and
> this is actually the counter we want to watch for here.
>
> Cc: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> ---
>  arch/powerpc/include/asm/mmu_context.h | 3 +++
>  arch/powerpc/mm/mmu_context_book3s64.c | 4 ----
>  2 files changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 9d2cd0c..24b590d 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -138,6 +138,9 @@ static inline void arch_dup_mmap(struct mm_struct *oldmm,
>
>  static inline void arch_exit_mmap(struct mm_struct *mm)
>  {
> +#ifdef CONFIG_SPAPR_TCE_IOMMU
> +       mm_iommu_cleanup(&mm->context);
> +#endif
>  }
>

The assumption is that all necessary tear down has happened. Earlier
we were doing the teardown
on the last mm ref drop, do we have any dependence on that? I don't
think so, but just checking

Does qemu dotce_iommu_register_pages on exit()/atexit() or do we rely
on the exit path to do the teardown?

Can we please remove the #ifdef CONFIG_SPAPR_TCE_IOMMU here and have a version
of mm_iommu_cleanup that handles it

Basically do

#ifdef CONFIG_SPAPR_TCE_IOMMU
extern void mm_iommu_cleanup(void *ptr)
#else
static inline mm_iommu_cleanup(void *ptr) {}
#endif

Otherwise looks good

Acked-by: Balbir Singh <bsingharora@xxxxxxxxx>