Re: [PATCH v2] vfio/type1: Remove locked page accounting workqueue

[Peter Xu]

On Thu, Apr 06, 2017 at 08:53:43AM -0600, 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 able to race the workqueue to enter more mappings
> than they're allowed.  It's not entirely clear what motivated this
> workqueue mechanism in the original vfio design, but it seems to
> introduce more problems than it solves, so remove it 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.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
> 
> v2: Fixed missed mmput on failure to acquire mmap_sem as noted by Eric
> 
>  drivers/vfio/vfio_iommu_type1.c |  101 ++++++++++++++++++---------------------
>  1 file changed, 46 insertions(+), 55 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 32d2633092a3..b799edbb8c4f 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -246,69 +246,45 @@ 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)
> +static int vfio_lock_acct(struct task_struct *task, long npage)
>  {
> -	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)
> -{
> -	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);

A question besides current patch: could I ask why we need to take
special care for is_current? I see that is only used to only try avoid
get_task_mm() when proper, but is get_task_mm() a heavy operation?

>  	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;
> +
> +			limit = is_current ?
> +				rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT :
> +				task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;

Maybe we can directly use task_rlimit() here? Since looks like
rlimit() is calling it as well, with "current".

> +
> +			if (mm->locked_vm + npage <= limit)
> +				mm->locked_vm += npage;
> +			else
> +				ret = -ENOMEM;
> +		}
>  
> -	if (is_current) {
> -		mm = get_task_mm(task);
> -		if (!mm)
> -			return;
> +		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 +381,7 @@ 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 +418,6 @@ 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 +434,25 @@ 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;
> +	}

The change in vfio_pin_pages_remote() seems to contain a functional
change totally not related to the subject (IIUC, we are going to unpin
those pages if the huge page can only be pinned partially, and we are
not doing that before)? If so, would it be nice to split current patch
into two, or at least mention this behavior change in commit log of
this patch?

Thanks,

>  
>  	return pinned;
>  }
> @@ -522,8 +507,14 @@ 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:
> 

-- 
Peter Xu