Re: [PATCH v4 1/2] vfio/type1: Remove locked page accounting workqueue

[Kirti Wankhede]

On 4/18/2017 12:49 AM, Alex Williamson wrote:
> On Tue, 18 Apr 2017 00:35:06 +0530
> Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote:
> 
>> On 4/17/2017 8:02 PM, Alex Williamson wrote:
>>> On Mon, 17 Apr 2017 14:47:54 +0800
>>> Peter Xu <peterx@xxxxxxxxxx> wrote:
>>>   
>>>> On Sun, Apr 16, 2017 at 07:42:27PM -0600, Alex Williamson wrote:
>>>>
>>>> [...]
>>>>  
>>>>> -static void vfio_lock_acct(struct task_struct *task, long npage)
>>>>> +static int vfio_lock_acct(struct task_struct *task, long npage, bool lock_cap)
>>>>>  {
>>>>> -	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 || lock_cap) {    
>>>>
>>>> Nit: maybe we can avoid passing in lock_cap in all the callers of
>>>> vfio_lock_acct() and fetch it via has_capability() only if npage < 0?
>>>> IMHO that'll keep the vfio_lock_acct() interface cleaner, and we won't
>>>> need to pass in "false" any time when doing unpins.  
>>>
>>> Unfortunately vfio_pin_pages_remote() needs to know about lock_cap
>>> since it tests whether the user is exceeding their locked memory
>>> limit.  The other callers could certainly get away with
>>> vfio_lock_acct() testing the capability itself but that would add a
>>> redundant call for the most common user.  I'm not a big fan of passing
>>> a lock_cap bool either, but it seemed the best fix for now.  The
>>> cleanest alternative I can up with is this (untested):
>>>   
>>
>> In my opinion, passing 'bool lock_cap' looks much clean and simple.
>>
>> Reviewed-by: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> 
> Well shoot, I was just starting to warm up to the bool*.  I like that
> we're not presuming the polarity for the callers we expect to be
> removing pages and I generally just dislike passing fixed bool
> parameters to change the function behavior.  I've cleaned it up a bit
> further and was starting to do some testing on this which I'd propose
> for v5.  Does it change your opinion?

If passing fixed bool parameter is the concern then I would lean towards
Peter's suggestion. vfio_pin_pages_remote() will check lock capability
outside vfio_lock_acct() and again in vfio_lock_acct(). At other places,
it will be takes care within vfio_lock_acct()

Thanks,
Kirti

> 
> commit cd61c5f507d614ac14b75b0a548c8738deff88ea
> Author: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Date:   Thu Apr 13 14:10:15 2017 -0600
> 
>     vfio/type1: Remove locked page accounting workqueue
>     
>     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>
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 32d2633092a3..a8a079ba9477 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -246,69 +246,46 @@ 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, bool *lock_cap)
>  {
> -	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) {
> +			if (lock_cap ? !*lock_cap :
> +			    !has_capability(task, CAP_IPC_LOCK)) {
> +				unsigned long limit;
> +
> +				limit = task_rlimit(task,
> +						RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +
> +				if (mm->locked_vm + npage > limit)
> +					ret = -ENOMEM;
> +			}
> +		}
> +
> +		if (!ret)
> +			mm->locked_vm += npage;
>  
> -	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 +382,7 @@ 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 +419,6 @@ 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 +435,25 @@ 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, &lock_cap);
> +
> +unpin_out:
> +	if (ret) {
> +		if (!rsvd) {
> +			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> +				put_pfn(pfn, dma->prot);
> +		}
> +
> +		return ret;
> +	}
>  
>  	return pinned;
>  }
> @@ -488,7 +474,7 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
>  	}
>  
>  	if (do_accounting)
> -		vfio_lock_acct(dma->task, locked - unlocked);
> +		vfio_lock_acct(dma->task, locked - unlocked, NULL);
>  
>  	return unlocked;
>  }
> @@ -522,8 +508,14 @@ 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, &lock_cap);
> +		if (ret) {
> +			put_pfn(*pfn_base, dma->prot);
> +			goto pin_page_exit;
> +		}
> +	}
> +
>  	ret = 1;
>  
>  pin_page_exit:
> @@ -543,7 +535,7 @@ static int vfio_unpin_page_external(struct vfio_dma *dma, dma_addr_t iova,
>  	unlocked = vfio_iova_put_vfio_pfn(dma, vpfn);
>  
>  	if (do_accounting)
> -		vfio_lock_acct(dma->task, -unlocked);
> +		vfio_lock_acct(dma->task, -unlocked, NULL);
>  
>  	return unlocked;
>  }
> @@ -740,7 +732,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>  
>  	dma->iommu_mapped = false;
>  	if (do_accounting) {
> -		vfio_lock_acct(dma->task, -unlocked);
> +		vfio_lock_acct(dma->task, -unlocked, NULL);
>  		return 0;
>  	}
>  	return unlocked;
> @@ -1382,7 +1374,7 @@ static void vfio_iommu_unmap_unpin_reaccount(struct vfio_iommu *iommu)
>  			if (!is_invalid_reserved_pfn(vpfn->pfn))
>  				locked++;
>  		}
> -		vfio_lock_acct(dma->task, locked - unlocked);
> +		vfio_lock_acct(dma->task, locked - unlocked, NULL);
>  	}
>  }
>  
> 
> Patch 2/2 would clearly change the &lock_cap in
> vfio_pin_page_external() to a NULL, so only _remote passes a pointer
> there.  Thanks,
> 
> Alex
>