Re: [PATCH 2/2] vhost-vdpa: fix page pinning leakage in error path (rework)

[Jason Wang]

On 2020/10/30 下午3:45, Si-Wei Liu wrote:
> Pinned pages are not properly accounted particularly when
> mapping error occurs on IOTLB update. Clean up dangling
> pinned pages for the error path.
>
> The memory usage for bookkeeping pinned pages is reverted
> to what it was before: only one single free page is needed.
> This helps reduce the host memory demand for VM with a large
> amount of memory, or in the situation where host is running
> short of free memory.
>
> Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> ---
>   drivers/vhost/vdpa.c | 64 +++++++++++++++++++++++++++++++++++++---------------
>   1 file changed, 46 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index b6d9016..8da8558 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -560,6 +560,8 @@ static int vhost_vdpa_map(struct vhost_vdpa *v,
>   
>   	if (r)
>   		vhost_iotlb_del_range(dev->iotlb, iova, iova + size - 1);
> +	else
> +		atomic64_add(size >> PAGE_SHIFT, &dev->mm->pinned_vm);
>   
>   	return r;
>   }
> @@ -591,14 +593,16 @@ static int vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
>   	unsigned long list_size = PAGE_SIZE / sizeof(struct page *);
>   	unsigned int gup_flags = FOLL_LONGTERM;
>   	unsigned long npages, cur_base, map_pfn, last_pfn = 0;
> -	unsigned long locked, lock_limit, pinned, i;
> +	unsigned long lock_limit, sz2pin, nchunks, i;
>   	u64 iova = msg->iova;
> +	long pinned;
>   	int ret = 0;
>   
>   	if (vhost_iotlb_itree_first(iotlb, msg->iova,
>   				    msg->iova + msg->size - 1))
>   		return -EEXIST;
>   
> +	/* Limit the use of memory for bookkeeping */
>   	page_list = (struct page **) __get_free_page(GFP_KERNEL);
>   	if (!page_list)
>   		return -ENOMEM;
> @@ -607,52 +611,64 @@ static int vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
>   		gup_flags |= FOLL_WRITE;
>   
>   	npages = PAGE_ALIGN(msg->size + (iova & ~PAGE_MASK)) >> PAGE_SHIFT;
> -	if (!npages)
> -		return -EINVAL;
> +	if (!npages) {
> +		ret = -EINVAL;
> +		goto free;
> +	}
>   
>   	mmap_read_lock(dev->mm);
>   
> -	locked = atomic64_add_return(npages, &dev->mm->pinned_vm);
>   	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> -
> -	if (locked > lock_limit) {
> +	if (npages + atomic64_read(&dev->mm->pinned_vm) > lock_limit) {
>   		ret = -ENOMEM;
> -		goto out;
> +		goto unlock;
>   	}
>   
>   	cur_base = msg->uaddr & PAGE_MASK;
>   	iova &= PAGE_MASK;
> +	nchunks = 0;
>   
>   	while (npages) {
> -		pinned = min_t(unsigned long, npages, list_size);
> -		ret = pin_user_pages(cur_base, pinned,
> -				     gup_flags, page_list, NULL);
> -		if (ret != pinned)
> +		sz2pin = min_t(unsigned long, npages, list_size);
> +		pinned = pin_user_pages(cur_base, sz2pin,
> +					gup_flags, page_list, NULL);
> +		if (sz2pin != pinned) {
> +			if (pinned < 0) {
> +				ret = pinned;
> +			} else {
> +				unpin_user_pages(page_list, pinned);
> +				ret = -ENOMEM;
> +			}
>   			goto out;
> +		}
> +		nchunks++;
>   
>   		if (!last_pfn)
>   			map_pfn = page_to_pfn(page_list[0]);
>   
> -		for (i = 0; i < ret; i++) {
> +		for (i = 0; i < pinned; i++) {
>   			unsigned long this_pfn = page_to_pfn(page_list[i]);
>   			u64 csize;
>   
>   			if (last_pfn && (this_pfn != last_pfn + 1)) {
>   				/* Pin a contiguous chunk of memory */
>   				csize = (last_pfn - map_pfn + 1) << PAGE_SHIFT;
> -				if (vhost_vdpa_map(v, iova, csize,
> -						   map_pfn << PAGE_SHIFT,
> -						   msg->perm))
> +				ret = vhost_vdpa_map(v, iova, csize,
> +						     map_pfn << PAGE_SHIFT,
> +						     msg->perm);
> +				if (ret)
>   					goto out;
> +
>   				map_pfn = this_pfn;
>   				iova += csize;
> +				nchunks = 0;
>   			}
>   
>   			last_pfn = this_pfn;
>   		}
>   
> -		cur_base += ret << PAGE_SHIFT;
> -		npages -= ret;
> +		cur_base += pinned << PAGE_SHIFT;
> +		npages -= pinned;
>   	}
>   
>   	/* Pin the rest chunk */
> @@ -660,10 +676,22 @@ static int vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
>   			     map_pfn << PAGE_SHIFT, msg->perm);
>   out:
>   	if (ret) {
> +		if (nchunks && last_pfn) {
> +			unsigned long pfn;
> +
> +			/*
> +			 * Unpin the outstanding pages which are unmapped.
> +			 * Mapped pages are accounted in vdpa_map(), thus
> +			 * will be handled by vdpa_unmap().
> +			 */
> +			for (pfn = map_pfn; pfn <= last_pfn; pfn++)
> +				unpin_user_page(pfn_to_page(pfn));
> +		}
>   		vhost_vdpa_unmap(v, msg->iova, msg->size);


I want to know what's wrong with current code.

We call vhost_vdpa_unmap() on error which calls vhost_vdpa_iotlb_unmap() 
that will unpin and reduce the pinned_vm.

Thanks


> -		atomic64_sub(npages, &dev->mm->pinned_vm);
>   	}
> +unlock:
>   	mmap_read_unlock(dev->mm);
> +free:
>   	free_page((unsigned long)page_list);
>   	return ret;
>   }