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

From: Alexey Kardashevskiy
Date: Thu Jul 14 2016 - 07:53:01 EST

On 14/07/16 20:12, Balbir Singh wrote:
> 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)
>> {
>> + 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?

QEMU does call unregister when a memory region is deleted. Handling it in
arch_exit_mmap() is for the case when a QEMU process just dies for whatever

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

That would be a change unrelated to the fix. And I just do not see the
point in this - mm_iommu_cleanup() is declared in the very same header.

> Basically do
> 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>