Subject: Re: [PATCH kernel v11 09/34] vfio: powerpc/spapr: Move locked_vm accounting to helpers

From: David Gibson <[email-masked]>
Date: Mon, 1 Jun 2015 14:28:14 +1000

On Fri, May 29, 2015 at 06:44:33PM +1000, Alexey Kardashevskiy wrote:
> There moves locked pages accounting to helpers.
> Later they will be reused for Dynamic DMA windows (DDW).
> 
> This reworks debug messages to show the current value and the limit.
> 
> This stores the locked pages number in the container so when unlocking
> the iommu table pointer won't be needed. This does not have an effect
> now but it will with the multiple tables per container as then we will
> allow attaching/detaching groups on fly and we may end up having
> a container with no group attached but with the counter incremented.
> 
> While we are here, update the comment explaining why RLIMIT_MEMLOCK
> might be required to be bigger than the guest RAM. This also prints
> pid of the current process in pr_warn/pr_debug.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> [aw: for the vfio related changes]
> Acked-by: Alex Williamson <[email-masked]>
> Reviewed-by: David Gibson <[email-masked]>
> Reviewed-by: Gavin Shan <[email-masked]>
> ---
> Changes:
> v4:
> * new helpers do nothing if @npages == 0
> * tce_iommu_disable() now can decrement the counter if the group was
> detached (not possible now but will be in the future)
> ---
>  drivers/vfio/vfio_iommu_spapr_tce.c | 82 ++++++++++++++++++++++++++++---------
>  1 file changed, 63 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index 64300cc..40583f9 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -29,6 +29,51 @@
>  static void tce_iommu_detach_group(void *iommu_data,
>  		struct iommu_group *iommu_group);
>  
> +static long try_increment_locked_vm(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;

Is there a possibility of userspace triggering an integer overflow
here, if npages is really huge?

> +	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +	if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> +		ret = -ENOMEM;
> +	else
> +		current->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,
> +			rlimit(RLIMIT_MEMLOCK),
> +			ret ? " - exceeded" : "");
> +
> +	up_write(&current->mm->mmap_sem);
> +
> +	return ret;
> +}
> +
> +static void decrement_locked_vm(long npages)
> +{
> +	if (!current || !current->mm || !npages)
> +		return; /* process exited */
> +
> +	down_write(&current->mm->mmap_sem);
> +	if (npages > current->mm->locked_vm)
> +		npages = current->mm->locked_vm;

Can this case ever occur (without there being a leak bug somewhere
else in the code)?

> +	current->mm->locked_vm -= npages;
> +	pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%ld\n", current->pid,
> +			npages << PAGE_SHIFT,
> +			current->mm->locked_vm << PAGE_SHIFT,
> +			rlimit(RLIMIT_MEMLOCK));
> +	up_write(&current->mm->mmap_sem);
> +}
> +
>  /*
>   * VFIO IOMMU fd for SPAPR_TCE IOMMU implementation
>   *
> @@ -45,6 +90,7 @@ struct tce_container {
>  	struct mutex lock;
>  	struct iommu_table *tbl;
>  	bool enabled;
> +	unsigned long locked_pages;
>  };
>  
>  static bool tce_page_is_contained(struct page *page, unsigned page_shift)
> @@ -60,7 +106,7 @@ static bool tce_page_is_contained(struct page *page, unsigned page_shift)
>  static int tce_iommu_enable(struct tce_container *container)
>  {
>  	int ret = 0;
> -	unsigned long locked, lock_limit, npages;
> +	unsigned long locked;
>  	struct iommu_table *tbl = container->tbl;
>  
>  	if (!container->tbl)
> @@ -89,21 +135,22 @@ static int tce_iommu_enable(struct tce_container *container)
>  	 * Also we don't have a nice way to fail on H_PUT_TCE due to ulimits,
>  	 * that would effectively kill the guest at random points, much better
>  	 * enforcing the limit based on the max that the guest can map.
> +	 *
> +	 * Unfortunately at the moment it counts whole tables, no matter how
> +	 * much memory the guest has. I.e. for 4GB guest and 4 IOMMU groups
> +	 * each with 2GB DMA window, 8GB will be counted here. The reason for
> +	 * this is that we cannot tell here the amount of RAM used by the guest
> +	 * as this information is only available from KVM and VFIO is
> +	 * KVM agnostic.
>  	 */
> -	down_write(&current->mm->mmap_sem);
> -	npages = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
> -	locked = current->mm->locked_vm + npages;
> -	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> -	if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
> -		pr_warn("RLIMIT_MEMLOCK (%ld) exceeded\n",
> -				rlimit(RLIMIT_MEMLOCK));
> -		ret = -ENOMEM;
> -	} else {
> +	locked = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
> +	ret = try_increment_locked_vm(locked);
> +	if (ret)
> +		return ret;
>  
> -		current->mm->locked_vm += npages;
> -		container->enabled = true;
> -	}
> -	up_write(&current->mm->mmap_sem);
> +	container->locked_pages = locked;
> +
> +	container->enabled = true;
>  
>  	return ret;
>  }
> @@ -115,13 +162,10 @@ static void tce_iommu_disable(struct tce_container *container)
>  
>  	container->enabled = false;
>  
> -	if (!container->tbl || !current->mm)
> +	if (!current->mm)
>  		return;
>  
> -	down_write(&current->mm->mmap_sem);
> -	current->mm->locked_vm -= (container->tbl->it_size <<
> -			container->tbl->it_page_shift) >> PAGE_SHIFT;
> -	up_write(&current->mm->mmap_sem);
> +	decrement_locked_vm(container->locked_pages);
>  }
>  
>  static void *tce_iommu_open(unsigned long arg)

-- 
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