Subject: Re: [PATCH kernel v3 3/4] vfio/spapr: Cache mm in tce_container

From: Nicholas Piggin <[email-masked]>
Date: Thu, 20 Oct 2016 18:31:21 +1100

On Thu, 20 Oct 2016 14:03:49 +1100
Alexey Kardashevskiy <[email-masked]> wrote:

> In some situations the userspace memory context may live longer than
> the userspace process itself so if we need to do proper memory context
> cleanup, we better cache @mm and use it later when the process is gone
> (@current or @current->mm is NULL).
> 
> This references mm and stores the pointer in the container; this is done
> when a container is just created so checking for !current->mm in other
> places becomes pointless.
> 
> This replaces current->mm with container->mm everywhere except debug
> prints.
> 
> This adds a check that current->mm is the same as the one stored in
> the container to prevent userspace from registering memory in other
> processes.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  drivers/vfio/vfio_iommu_spapr_tce.c | 127 ++++++++++++++++++++----------------
>  1 file changed, 71 insertions(+), 56 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index d0c38b2..6b0b121 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -31,49 +31,46 @@

Does it make sense to move the rest of these hunks into patch 2?
I think they're similarly just moving the mm reference into callers.

>  static void tce_iommu_detach_group(void *iommu_data,
>  		struct iommu_group *iommu_group);
>  
> -static long try_increment_locked_vm(long npages)
> +static long try_increment_locked_vm(struct mm_struct *mm, long npages)
>  {
>  	long ret = 0, locked, lock_limit;
>  
> -	if (!current || !current->mm)
> -		return -ESRCH; /* process exited */
> -
>  	if (!npages)
>  		return 0;
>  
> -	down_write(&current->mm->mmap_sem);
> -	locked = current->mm->locked_vm + npages;
> +	down_write(&mm->mmap_sem);
> +	locked = mm->locked_vm + npages;
>  	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>  	if (locked > lock_limit && !capable(CAP_IPC_LOCK))
>  		ret = -ENOMEM;
>  	else
> -		current->mm->locked_vm += npages;
> +		mm->locked_vm += npages;
>  
>  	pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%ld%s\n", current->pid,
>  			npages << PAGE_SHIFT,
> -			current->mm->locked_vm << PAGE_SHIFT,
> +			mm->locked_vm << PAGE_SHIFT,
>  			rlimit(RLIMIT_MEMLOCK),
>  			ret ? " - exceeded" : "");
>  
> -	up_write(&current->mm->mmap_sem);
> +	up_write(&mm->mmap_sem);
>  
>  	return ret;
>  }
>  
> -static void decrement_locked_vm(long npages)
> +static void decrement_locked_vm(struct mm_struct *mm, long npages)
>  {
> -	if (!current || !current->mm || !npages)
> +	if (!mm || !npages)
>  		return; /* process exited */

I know you're trying to be defensive and change as little logic as possible,
but some cases should be an error, and I think some of the "process exited"
comments were wrong anyway.

Maybe pull the !mm test into the caller and make it WARN_ON?

> @@ -317,6 +311,9 @@ static void *tce_iommu_open(unsigned long arg)
>  		return ERR_PTR(-EINVAL);
>  	}
>  
> +	if (!current->mm)
> +		return ERR_PTR(-ESRCH); /* process exited */

A userspace thread in the kernel can't have its mm disappear, unless you
are actually in the exit code. !current->mm is more like a test for a kernel
thread.

> +
>  	container = kzalloc(sizeof(*container), GFP_KERNEL);
>  	if (!container)
>  		return ERR_PTR(-ENOMEM);
> @@ -326,13 +323,17 @@ static void *tce_iommu_open(unsigned long arg)
>  
>  	container->v2 = arg == VFIO_SPAPR_TCE_v2_IOMMU;
>  
> +	container->mm = current->mm;
> +	atomic_inc(&container->mm->mm_count);
> +
>  	return container;

It's a nitpick if you respin the patch, but I guess it would better be
described as a reference than a cache of the object. "have tce_container
take a reference to mm_struct".

> @@ -515,13 +526,16 @@ static long tce_iommu_build_v2(struct tce_container *container,
>  	unsigned long hpa;
>  	enum dma_data_direction dirtmp;
>  
> +	if (container->mm != current->mm)
> +		return -ESRCH;

Good, is this condition now enforced on all entrypoints that use
container->mm (except the final teardown)? (The mlock/rlimit stuff,
as we talked about before, doesn't make sense if not).

Thanks,
Nick