Subject: Re: [PATCH kernel v4 4/4] powerpc/mm/iommu, vfio/spapr: Put pages on VFIO container shutdown

From: David Gibson <[email-masked]>
Date: Mon, 31 Oct 2016 15:23:47 +1100

On Mon, Oct 31, 2016 at 03:13:21PM +1100, Alexey Kardashevskiy wrote:
> On 31/10/16 14:13, David Gibson wrote:
> > On Tue, Oct 25, 2016 at 03:55:56PM +1100, Alexey Kardashevskiy wrote:
> >> On 25/10/16 15:44, David Gibson wrote:
> >>> On Mon, Oct 24, 2016 at 05:53:10PM +1100, Alexey Kardashevskiy wrote:
> >>>> At the moment the userspace tool is expected to request pinning of
> >>>> the entire guest RAM when VFIO IOMMU SPAPR v2 driver is present.
> >>>> 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 moves preregistered regions tracking from MM to VFIO; insteads of
> >>>> using mm_iommu_table_group_mem_t::used, tce_container::prereg_list is
> >>>> added so each container releases regions which it has pre-registered.
> >>>>
> >>>> This changes the userspace interface to return EBUSY if a memory
> >>>> region is already registered in a container. However it should not
> >>>> have any practical effect as the only userspace tool available now
> >>>> does register memory region 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 <[email-masked]>
> >>>> Reviewed-by: Nicholas Piggin <[email-masked]>
> >>>
> >>> On the grounds that this leaves things in a better state than before:
> >>>
> >>> Reviewed-by: David Gibson <[email-masked]>
> >>>
> >>> On the other hand the implementation is kind of clunky, with the way
> >>> it keeps the mm-level and vfio-level lists of regions in parallel.
> >>> With this change, does the mm-level list actually serve any purpose at
> >>> all, or could it all be moved into the vfio-level list?
> >>
> >>
> >> The mm-level list allows not having gup() called for each container (minor
> >> thing, I suppose) and it also tracks a number of active mappings which will
> >> become useful when we add in-kernel real-mode TCE acceleration as
> >> vfio-level code cannot run in realmode.
> > 
> > Hm, ok.  So, if two different containers pre-register the same region
> > of memory, IIUC in the proposed code, the region will get one entry in
> > the mm level list, and that entry will be referenced in the lists for
> > both containers.  Yes?
> 
> Yes.
> 
> 
> > What happens if two different containers try to pre-register
> > different, but overlapping, mm regions?
> 
> The second container will fail to preregister memory - mm_iommu_get() will
> return -EINVAL.

Um.. yeah.. that's not really ok.  Prohibiting overlapping
registrations on the same container is reasonable enough.  Having a
container not be able to register memory because some completely
different container has registered something overlapping is getting
very ugly.

> I am wondering what happens to the series now.
> 
> Alex, could you please have a look and comment? Thanks.
> 
> 
> 
> > 
> >>
> >>
> >>
> >>>
> >>>> ---
> >>>> Changes:
> >>>> v4:
> >>>> * changed tce_iommu_register_pages() to call mm_iommu_find() first and
> >>>> avoid calling mm_iommu_put() if memory is preregistered already
> >>>>
> >>>> v3:
> >>>> * moved tce_iommu_prereg_free() call out of list_for_each_entry()
> >>>>
> >>>> v2:
> >>>> * updated commit log
> >>>> ---
> >>>>  arch/powerpc/mm/mmu_context_book3s64.c |  4 ---
> >>>>  arch/powerpc/mm/mmu_context_iommu.c    | 11 -------
> >>>>  drivers/vfio/vfio_iommu_spapr_tce.c    | 58 +++++++++++++++++++++++++++++++++-
> >>>>  3 files changed, 57 insertions(+), 16 deletions(-)
> >>>>
> >>>> diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
> >>>> index ad82735..1a07969 100644
> >>>> --- a/arch/powerpc/mm/mmu_context_book3s64.c
> >>>> +++ b/arch/powerpc/mm/mmu_context_book3s64.c
> >>>> @@ -159,10 +159,6 @@ static inline void destroy_pagetable_page(struct mm_struct *mm)
> >>>>  
> >>>>  void destroy_context(struct mm_struct *mm)
> >>>>  {
> >>>> -#ifdef CONFIG_SPAPR_TCE_IOMMU
> >>>> -	mm_iommu_cleanup(mm);
> >>>> -#endif
> >>>> -
> >>>>  #ifdef CONFIG_PPC_ICSWX
> >>>>  	drop_cop(mm->context.acop, mm);
> >>>>  	kfree(mm->context.cop_lockp);
> >>>> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> >>>> index 4c6db09..104bad0 100644
> >>>> --- a/arch/powerpc/mm/mmu_context_iommu.c
> >>>> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> >>>> @@ -365,14 +365,3 @@ void mm_iommu_init(struct mm_struct *mm)
> >>>>  {
> >>>>  	INIT_LIST_HEAD_RCU(&mm->context.iommu_group_mem_list);
> >>>>  }
> >>>> -
> >>>> -void mm_iommu_cleanup(struct mm_struct *mm)
> >>>> -{
> >>>> -	struct mm_iommu_table_group_mem_t *mem, *tmp;
> >>>> -
> >>>> -	list_for_each_entry_safe(mem, tmp, &mm->context.iommu_group_mem_list,
> >>>> -			next) {
> >>>> -		list_del_rcu(&mem->next);
> >>>> -		mm_iommu_do_free(mem);
> >>>> -	}
> >>>> -}
> >>>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> index 81ab93f..001a488 100644
> >>>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> @@ -86,6 +86,15 @@ struct tce_iommu_group {
> >>>>  };
> >>>>  
> >>>>  /*
> >>>> + * A container needs to remember which preregistered region  it has
> >>>> + * referenced to do proper cleanup at the userspace process exit.
> >>>> + */
> >>>> +struct tce_iommu_prereg {
> >>>> +	struct list_head next;
> >>>> +	struct mm_iommu_table_group_mem_t *mem;
> >>>> +};
> >>>> +
> >>>> +/*
> >>>>   * The container descriptor supports only a single group per container.
> >>>>   * Required by the API as the container is not supplied with the IOMMU group
> >>>>   * at the moment of initialization.
> >>>> @@ -98,12 +107,27 @@ struct tce_container {
> >>>>  	struct mm_struct *mm;
> >>>>  	struct iommu_table *tables[IOMMU_TABLE_GROUP_MAX_TABLES];
> >>>>  	struct list_head group_list;
> >>>> +	struct list_head prereg_list;
> >>>>  };
> >>>>  
> >>>> +static long tce_iommu_prereg_free(struct tce_container *container,
> >>>> +		struct tce_iommu_prereg *tcemem)
> >>>> +{
> >>>> +	long ret;
> >>>> +
> >>>> +	list_del(&tcemem->next);
> >>>> +	ret = mm_iommu_put(container->mm, tcemem->mem);
> >>>> +	kfree(tcemem);
> >>>> +
> >>>> +	return ret;
> >>>> +}
> >>>> +
> >>>>  static long tce_iommu_unregister_pages(struct tce_container *container,
> >>>>  		__u64 vaddr, __u64 size)
> >>>>  {
> >>>>  	struct mm_iommu_table_group_mem_t *mem;
> >>>> +	struct tce_iommu_prereg *tcemem;
> >>>> +	bool found = false;
> >>>>  
> >>>>  	if ((vaddr & ~PAGE_MASK) || (size & ~PAGE_MASK))
> >>>>  		return -EINVAL;
> >>>> @@ -112,7 +136,17 @@ static long tce_iommu_unregister_pages(struct tce_container *container,
> >>>>  	if (!mem)
> >>>>  		return -ENOENT;
> >>>>  
> >>>> -	return mm_iommu_put(container->mm, mem);
> >>>> +	list_for_each_entry(tcemem, &container->prereg_list, next) {
> >>>> +		if (tcemem->mem == mem) {
> >>>> +			found = true;
> >>>> +			break;
> >>>> +		}
> >>>> +	}
> >>>> +
> >>>> +	if (!found)
> >>>> +		return -ENOENT;
> >>>> +
> >>>> +	return tce_iommu_prereg_free(container, tcemem);
> >>>>  }
> >>>>  
> >>>>  static long tce_iommu_register_pages(struct tce_container *container,
> >>>> @@ -120,16 +154,29 @@ static long tce_iommu_register_pages(struct tce_container *container,
> >>>>  {
> >>>>  	long ret = 0;
> >>>>  	struct mm_iommu_table_group_mem_t *mem = NULL;
> >>>> +	struct tce_iommu_prereg *tcemem;
> >>>>  	unsigned long entries = size >> PAGE_SHIFT;
> >>>>  
> >>>>  	if ((vaddr & ~PAGE_MASK) || (size & ~PAGE_MASK) ||
> >>>>  			((vaddr + size) < vaddr))
> >>>>  		return -EINVAL;
> >>>>  
> >>>> +	mem = mm_iommu_find(container->mm, vaddr, entries);
> >>>> +	if (mem) {
> >>>> +		list_for_each_entry(tcemem, &container->prereg_list, next) {
> >>>> +			if (tcemem->mem == mem)
> >>>> +				return -EBUSY;
> >>>> +		}
> >>>> +	}
> >>>> +
> >>>>  	ret = mm_iommu_get(container->mm, vaddr, entries, &mem);
> >>>>  	if (ret)
> >>>>  		return ret;
> >>>>  
> >>>> +	tcemem = kzalloc(sizeof(*tcemem), GFP_KERNEL);
> >>>> +	tcemem->mem = mem;
> >>>> +	list_add(&tcemem->next, &container->prereg_list);
> >>>> +
> >>>>  	container->enabled = true;
> >>>>  
> >>>>  	return 0;
> >>>> @@ -311,6 +358,7 @@ static void *tce_iommu_open(unsigned long arg)
> >>>>  
> >>>>  	mutex_init(&container->lock);
> >>>>  	INIT_LIST_HEAD_RCU(&container->group_list);
> >>>> +	INIT_LIST_HEAD_RCU(&container->prereg_list);
> >>>>  
> >>>>  	container->v2 = arg == VFIO_SPAPR_TCE_v2_IOMMU;
> >>>>  
> >>>> @@ -353,6 +401,14 @@ static void tce_iommu_release(void *iommu_data)
> >>>>  		tce_iommu_free_table(container, tbl);
> >>>>  	}
> >>>>  
> >>>> +	while (!list_empty(&container->prereg_list)) {
> >>>> +		struct tce_iommu_prereg *tcemem;
> >>>> +
> >>>> +		tcemem = list_first_entry(&container->prereg_list,
> >>>> +				struct tce_iommu_prereg, next);
> >>>> +		tce_iommu_prereg_free(container, tcemem);
> >>>> +	}
> >>>> +
> >>>>  	tce_iommu_disable(container);
> >>>>  	mmdrop(container->mm);
> >>>>  	mutex_destroy(&container->lock);
> >>>
> >>
> >>
> > 
> > 
> > 
> > 
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson