Re: [PATCH 2/5] vfio/spapr_tce: use pinned_vm instead of locked_vm to account pinned pages

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 12/02/2019 09:44, Daniel Jordan wrote:
> Beginning with bc3e53f682d9 ("mm: distinguish between mlocked and pinned
> pages"), locked and pinned pages are accounted separately.  The SPAPR
> TCE VFIO IOMMU driver accounts pinned pages to locked_vm; use pinned_vm
> instead.
> 
> pinned_vm recently became atomic and so no longer relies on mmap_sem
> held as writer: delete.
> 
> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> ---
>  Documentation/vfio.txt              |  6 +--
>  drivers/vfio/vfio_iommu_spapr_tce.c | 64 ++++++++++++++---------------
>  2 files changed, 33 insertions(+), 37 deletions(-)
> 
> diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
> index f1a4d3c3ba0b..fa37d65363f9 100644
> --- a/Documentation/vfio.txt
> +++ b/Documentation/vfio.txt
> @@ -308,7 +308,7 @@ This implementation has some specifics:
>     currently there is no way to reduce the number of calls. In order to make
>     things faster, the map/unmap handling has been implemented in real mode
>     which provides an excellent performance which has limitations such as
> -   inability to do locked pages accounting in real time.
> +   inability to do pinned pages accounting in real time.
>  
>  4) According to sPAPR specification, A Partitionable Endpoint (PE) is an I/O
>     subtree that can be treated as a unit for the purposes of partitioning and
> @@ -324,7 +324,7 @@ This implementation has some specifics:
>  		returns the size and the start of the DMA window on the PCI bus.
>  
>  	VFIO_IOMMU_ENABLE
> -		enables the container. The locked pages accounting
> +		enables the container. The pinned pages accounting
>  		is done at this point. This lets user first to know what
>  		the DMA window is and adjust rlimit before doing any real job.
>  
> @@ -454,7 +454,7 @@ This implementation has some specifics:
>  
>     PPC64 paravirtualized guests generate a lot of map/unmap requests,
>     and the handling of those includes pinning/unpinning pages and updating
> -   mm::locked_vm counter to make sure we do not exceed the rlimit.
> +   mm::pinned_vm counter to make sure we do not exceed the rlimit.
>     The v2 IOMMU splits accounting and pinning into separate operations:
>  
>     - VFIO_IOMMU_SPAPR_REGISTER_MEMORY/VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY ioctls
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index c424913324e3..f47e020dc5e4 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -34,9 +34,11 @@
>  static void tce_iommu_detach_group(void *iommu_data,
>  		struct iommu_group *iommu_group);
>  
> -static long try_increment_locked_vm(struct mm_struct *mm, long npages)
> +static long try_increment_pinned_vm(struct mm_struct *mm, long npages)
>  {
> -	long ret = 0, locked, lock_limit;
> +	long ret = 0;
> +	s64 pinned;
> +	unsigned long lock_limit;
>  
>  	if (WARN_ON_ONCE(!mm))
>  		return -EPERM;
> @@ -44,39 +46,33 @@ static long try_increment_locked_vm(struct mm_struct *mm, long npages)
>  	if (!npages)
>  		return 0;
>  
> -	down_write(&mm->mmap_sem);
> -	locked = mm->locked_vm + npages;
> +	pinned = atomic64_add_return(npages, &mm->pinned_vm);
>  	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> -	if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> +	if (pinned > lock_limit && !capable(CAP_IPC_LOCK)) {
>  		ret = -ENOMEM;
> -	else
> -		mm->locked_vm += npages;
> +		atomic64_sub(npages, &mm->pinned_vm);
> +	}
>  
> -	pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%ld%s\n", current->pid,
> +	pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%lu%s\n", current->pid,
>  			npages << PAGE_SHIFT,
> -			mm->locked_vm << PAGE_SHIFT,
> -			rlimit(RLIMIT_MEMLOCK),
> -			ret ? " - exceeded" : "");
> -
> -	up_write(&mm->mmap_sem);
> +			atomic64_read(&mm->pinned_vm) << PAGE_SHIFT,
> +			rlimit(RLIMIT_MEMLOCK), ret ? " - exceeded" : "");
>  
>  	return ret;
>  }
>  
> -static void decrement_locked_vm(struct mm_struct *mm, long npages)
> +static void decrement_pinned_vm(struct mm_struct *mm, long npages)
>  {
>  	if (!mm || !npages)
>  		return;
>  
> -	down_write(&mm->mmap_sem);
> -	if (WARN_ON_ONCE(npages > mm->locked_vm))
> -		npages = mm->locked_vm;
> -	mm->locked_vm -= npages;
> -	pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%ld\n", current->pid,
> +	if (WARN_ON_ONCE(npages > atomic64_read(&mm->pinned_vm)))
> +		npages = atomic64_read(&mm->pinned_vm);
> +	atomic64_sub(npages, &mm->pinned_vm);
> +	pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%lu\n", current->pid,
>  			npages << PAGE_SHIFT,
> -			mm->locked_vm << PAGE_SHIFT,
> +			atomic64_read(&mm->pinned_vm) << PAGE_SHIFT,
>  			rlimit(RLIMIT_MEMLOCK));
> -	up_write(&mm->mmap_sem);


So it used to be down_write+up_write and stuff in between.

Now it is 3 independent accesses (actually 4 but the last one is
diagnostic) with no locking around them. Why do not we need a lock
anymore precisely? Thanks,




>  }
>  
>  /*
> @@ -110,7 +106,7 @@ struct tce_container {
>  	bool enabled;
>  	bool v2;
>  	bool def_window_pending;
> -	unsigned long locked_pages;
> +	unsigned long pinned_pages;
>  	struct mm_struct *mm;
>  	struct iommu_table *tables[IOMMU_TABLE_GROUP_MAX_TABLES];
>  	struct list_head group_list;
> @@ -283,7 +279,7 @@ static int tce_iommu_find_free_table(struct tce_container *container)
>  static int tce_iommu_enable(struct tce_container *container)
>  {
>  	int ret = 0;
> -	unsigned long locked;
> +	unsigned long pinned;
>  	struct iommu_table_group *table_group;
>  	struct tce_iommu_group *tcegrp;
>  
> @@ -292,15 +288,15 @@ static int tce_iommu_enable(struct tce_container *container)
>  
>  	/*
>  	 * When userspace pages are mapped into the IOMMU, they are effectively
> -	 * locked memory, so, theoretically, we need to update the accounting
> -	 * of locked pages on each map and unmap.  For powerpc, the map unmap
> +	 * pinned memory, so, theoretically, we need to update the accounting
> +	 * of pinned pages on each map and unmap.  For powerpc, the map unmap
>  	 * paths can be very hot, though, and the accounting would kill
>  	 * performance, especially since it would be difficult to impossible
>  	 * to handle the accounting in real mode only.
>  	 *
>  	 * To address that, rather than precisely accounting every page, we
> -	 * instead account for a worst case on locked memory when the iommu is
> -	 * enabled and disabled.  The worst case upper bound on locked memory
> +	 * instead account for a worst case on pinned memory when the iommu is
> +	 * enabled and disabled.  The worst case upper bound on pinned memory
>  	 * is the size of the whole iommu window, which is usually relatively
>  	 * small (compared to total memory sizes) on POWER hardware.
>  	 *
> @@ -317,7 +313,7 @@ static int tce_iommu_enable(struct tce_container *container)
>  	 *
>  	 * So we do not allow enabling a container without a group attached
>  	 * as there is no way to know how much we should increment
> -	 * the locked_vm counter.
> +	 * the pinned_vm counter.
>  	 */
>  	if (!tce_groups_attached(container))
>  		return -ENODEV;
> @@ -335,12 +331,12 @@ static int tce_iommu_enable(struct tce_container *container)
>  	if (ret)
>  		return ret;
>  
> -	locked = table_group->tce32_size >> PAGE_SHIFT;
> -	ret = try_increment_locked_vm(container->mm, locked);
> +	pinned = table_group->tce32_size >> PAGE_SHIFT;
> +	ret = try_increment_pinned_vm(container->mm, pinned);
>  	if (ret)
>  		return ret;
>  
> -	container->locked_pages = locked;
> +	container->pinned_pages = pinned;
>  
>  	container->enabled = true;
>  
> @@ -355,7 +351,7 @@ static void tce_iommu_disable(struct tce_container *container)
>  	container->enabled = false;
>  
>  	BUG_ON(!container->mm);
> -	decrement_locked_vm(container->mm, container->locked_pages);
> +	decrement_pinned_vm(container->mm, container->pinned_pages);
>  }
>  
>  static void *tce_iommu_open(unsigned long arg)
> @@ -658,7 +654,7 @@ static long tce_iommu_create_table(struct tce_container *container,
>  	if (!table_size)
>  		return -EINVAL;
>  
> -	ret = try_increment_locked_vm(container->mm, table_size >> PAGE_SHIFT);
> +	ret = try_increment_pinned_vm(container->mm, table_size >> PAGE_SHIFT);
>  	if (ret)
>  		return ret;
>  
> @@ -677,7 +673,7 @@ static void tce_iommu_free_table(struct tce_container *container,
>  	unsigned long pages = tbl->it_allocated_size >> PAGE_SHIFT;
>  
>  	iommu_tce_table_put(tbl);
> -	decrement_locked_vm(container->mm, pages);
> +	decrement_pinned_vm(container->mm, pages);
>  }
>  
>  static long tce_iommu_create_window(struct tce_container *container,
> 

-- 
Alexey