Re: [PATCH v3] vfio/type1: Remove locked page accounting workqueue

[Kirti Wankhede]

On 4/12/2017 12:58 AM, Alex Williamson wrote:
> If the mmap_sem is contented then the vfio type1 IOMMU backend will
> defer locked page accounting updates to a workqueue task.  This has a
> few problems and depending on which side the user tries to play, they
> might be over-penalized for unmaps that haven't yet been accounted or
> race the workqueue to enter more mappings than they're allowed.  The
> original intent of this workqueue mechanism seems to be focused on
> reducing latency through the ioctl, but we cannot do so at the cost
> of correctness.  Remove this workqueue mechanism and update the
> callers to allow for failure.  We can also now recheck the limit under
> write lock to make sure we don't exceed it.
> 
> vfio_pin_pages_remote() also now necessarily includes an unwind path
> which we can jump to directly if the consecutive page pinning finds
> that we're exceeding the user's memory limits.  This avoids the
> current lazy approach which does accounting and mapping up to the
> fault, only to return an error on the next iteration to unwind the
> entire vfio_dma.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
> 
> v3: Update for comments from Peter
>     - Use task_rlimit() exclusively
>     - Discuss vfio_pin_pages_remote() exit branch in commitlog
> 
>  drivers/vfio/vfio_iommu_type1.c |   99 +++++++++++++++++----------------------
>  1 file changed, 44 insertions(+), 55 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 32d2633092a3..176ebcc0ffa2 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -246,69 +246,43 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
>  	return ret;
>  }
>  
> -struct vwork {
> -	struct mm_struct	*mm;
> -	long			npage;
> -	struct work_struct	work;
> -};
> -
> -/* delayed decrement/increment for locked_vm */
> -static void vfio_lock_acct_bg(struct work_struct *work)
> -{
> -	struct vwork *vwork = container_of(work, struct vwork, work);
> -	struct mm_struct *mm;
> -
> -	mm = vwork->mm;
> -	down_write(&mm->mmap_sem);
> -	mm->locked_vm += vwork->npage;
> -	up_write(&mm->mmap_sem);
> -	mmput(mm);
> -	kfree(vwork);
> -}
> -
> -static void vfio_lock_acct(struct task_struct *task, long npage)
> +static int vfio_lock_acct(struct task_struct *task, long npage)
>  {
> -	struct vwork *vwork;
>  	struct mm_struct *mm;
>  	bool is_current;
> +	int ret;
>  
>  	if (!npage)
> -		return;
> +		return 0;
>  
>  	is_current = (task->mm == current->mm);
>  
>  	mm = is_current ? task->mm : get_task_mm(task);
>  	if (!mm)
> -		return; /* process exited */
> +		return -ESRCH; /* process exited */
>  
> -	if (down_write_trylock(&mm->mmap_sem)) {
> -		mm->locked_vm += npage;
> -		up_write(&mm->mmap_sem);
> -		if (!is_current)
> -			mmput(mm);
> -		return;
> -	}
> +	ret = down_write_killable(&mm->mmap_sem);
> +	if (!ret) {
> +		if (npage < 0) {
> +			mm->locked_vm += npage;
> +		} else {
> +			unsigned long limit;
>  
> -	if (is_current) {
> -		mm = get_task_mm(task);
> -		if (!mm)
> -			return;
> +			limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +
> +			if (mm->locked_vm + npage <= limit)
> +				mm->locked_vm += npage;
> +			else
> +				ret = -ENOMEM;
> +		}
> +

Sorry if I'm late here on my review.

There are rlimit checks before calling vfio_lock_acct() while pinning
pages. I agree this is checked holding locks, so this check is more
robust, but still it feels redundant. I think you can remove checks from
vfio_pin_page_external() and vfio_pin_pages_remote().
Also while checking the limit, !lock_cap checks is not considered here.
That would mean that there code would impose limit check even without
lock capability?

Thanks,
Kirti


> +		up_write(&mm->mmap_sem);
>  	}
>  
> -	/*
> -	 * Couldn't get mmap_sem lock, so must setup to update
> -	 * mm->locked_vm later. If locked_vm were atomic, we
> -	 * wouldn't need this silliness
> -	 */
> -	vwork = kmalloc(sizeof(struct vwork), GFP_KERNEL);
> -	if (WARN_ON(!vwork)) {
> +	if (!is_current)
>  		mmput(mm);
> -		return;
> -	}
> -	INIT_WORK(&vwork->work, vfio_lock_acct_bg);
> -	vwork->mm = mm;
> -	vwork->npage = npage;
> -	schedule_work(&vwork->work);
> +
> +	return ret;
>  }
>  
>  /*
> @@ -405,7 +379,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  				  long npage, unsigned long *pfn_base)
>  {
> -	unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +	unsigned long pfn = 0, limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>  	bool lock_cap = capable(CAP_IPC_LOCK);
>  	long ret, pinned = 0, lock_acct = 0;
>  	bool rsvd;
> @@ -442,8 +416,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  	/* Lock all the consecutive pages from pfn_base */
>  	for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; pinned < npage;
>  	     pinned++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
> -		unsigned long pfn = 0;
> -
>  		ret = vaddr_get_pfn(current->mm, vaddr, dma->prot, &pfn);
>  		if (ret)
>  			break;
> @@ -460,14 +432,25 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  				put_pfn(pfn, dma->prot);
>  				pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
>  					__func__, limit << PAGE_SHIFT);
> -				break;
> +				ret = -ENOMEM;
> +				goto unpin_out;
>  			}
>  			lock_acct++;
>  		}
>  	}
>  
>  out:
> -	vfio_lock_acct(current, lock_acct);
> +	ret = vfio_lock_acct(current, lock_acct);
> +
> +unpin_out:
> +	if (ret) {
> +		if (!rsvd) {
> +			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> +				put_pfn(pfn, dma->prot);
> +		}
> +
> +		return ret;
> +	}
>  
>  	return pinned;
>  }
> @@ -522,8 +505,14 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
>  		goto pin_page_exit;
>  	}
>  
> -	if (!rsvd && do_accounting)
> -		vfio_lock_acct(dma->task, 1);
> +	if (!rsvd && do_accounting) {
> +		ret = vfio_lock_acct(dma->task, 1);
> +		if (ret) {
> +			put_pfn(*pfn_base, dma->prot);
> +			goto pin_page_exit;
> +		}
> +	}
> +
>  	ret = 1;
>  
>  pin_page_exit:
>