Re: [PATCH v2] virtiofs: use GFP_NOFS when enqueuing request through kworker

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jan 05, 2024 at 06:53:05PM +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> When invoking virtio_fs_enqueue_req() through kworker, both the
> allocation of the sg array and the bounce buffer still use GFP_ATOMIC.
> Considering the size of both the sg array and the bounce buffer may be
> greater than PAGE_SIZE, use GFP_NOFS instead of GFP_ATOMIC to lower the
> possibility of memory allocation failure.
> 

What's the practical benefit of this patch. Looks like if memory
allocation fails, we keep retrying at interval of 1ms and don't
return error to user space.

Thanks
Vivek

> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
> Change log:
> v2:
>   * pass gfp_t instead of bool to virtio_fs_enqueue_req() (Suggested by Matthew)
> 
> v1: https://lore.kernel.org/linux-fsdevel/20240104015805.2103766-1-houtao@huaweicloud.com
> 
>  fs/fuse/virtio_fs.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 3aac31d451985..8cf518624ce9e 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -87,7 +87,8 @@ struct virtio_fs_req_work {
>  };
>  
>  static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
> -				 struct fuse_req *req, bool in_flight);
> +				 struct fuse_req *req, bool in_flight,
> +				 gfp_t gfp);
>  
>  static const struct constant_table dax_param_enums[] = {
>  	{"always",	FUSE_DAX_ALWAYS },
> @@ -383,7 +384,7 @@ static void virtio_fs_request_dispatch_work(struct work_struct *work)
>  		list_del_init(&req->list);
>  		spin_unlock(&fsvq->lock);
>  
> -		ret = virtio_fs_enqueue_req(fsvq, req, true);
> +		ret = virtio_fs_enqueue_req(fsvq, req, true, GFP_NOFS);
>  		if (ret < 0) {
>  			if (ret == -ENOMEM || ret == -ENOSPC) {
>  				spin_lock(&fsvq->lock);
> @@ -488,7 +489,7 @@ static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
>  }
>  
>  /* Allocate and copy args into req->argbuf */
> -static int copy_args_to_argbuf(struct fuse_req *req)
> +static int copy_args_to_argbuf(struct fuse_req *req, gfp_t gfp)
>  {
>  	struct fuse_args *args = req->args;
>  	unsigned int offset = 0;
> @@ -502,7 +503,7 @@ static int copy_args_to_argbuf(struct fuse_req *req)
>  	len = fuse_len_args(num_in, (struct fuse_arg *) args->in_args) +
>  	      fuse_len_args(num_out, args->out_args);
>  
> -	req->argbuf = kmalloc(len, GFP_ATOMIC);
> +	req->argbuf = kmalloc(len, gfp);
>  	if (!req->argbuf)
>  		return -ENOMEM;
>  
> @@ -1119,7 +1120,8 @@ static unsigned int sg_init_fuse_args(struct scatterlist *sg,
>  
>  /* Add a request to a virtqueue and kick the device */
>  static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
> -				 struct fuse_req *req, bool in_flight)
> +				 struct fuse_req *req, bool in_flight,
> +				 gfp_t gfp)
>  {
>  	/* requests need at least 4 elements */
>  	struct scatterlist *stack_sgs[6];
> @@ -1140,8 +1142,8 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
>  	/* Does the sglist fit on the stack? */
>  	total_sgs = sg_count_fuse_req(req);
>  	if (total_sgs > ARRAY_SIZE(stack_sgs)) {
> -		sgs = kmalloc_array(total_sgs, sizeof(sgs[0]), GFP_ATOMIC);
> -		sg = kmalloc_array(total_sgs, sizeof(sg[0]), GFP_ATOMIC);
> +		sgs = kmalloc_array(total_sgs, sizeof(sgs[0]), gfp);
> +		sg = kmalloc_array(total_sgs, sizeof(sg[0]), gfp);
>  		if (!sgs || !sg) {
>  			ret = -ENOMEM;
>  			goto out;
> @@ -1149,7 +1151,7 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
>  	}
>  
>  	/* Use a bounce buffer since stack args cannot be mapped */
> -	ret = copy_args_to_argbuf(req);
> +	ret = copy_args_to_argbuf(req, gfp);
>  	if (ret < 0)
>  		goto out;
>  
> @@ -1245,7 +1247,7 @@ __releases(fiq->lock)
>  		 fuse_len_args(req->args->out_numargs, req->args->out_args));
>  
>  	fsvq = &fs->vqs[queue_id];
> -	ret = virtio_fs_enqueue_req(fsvq, req, false);
> +	ret = virtio_fs_enqueue_req(fsvq, req, false, GFP_ATOMIC);
>  	if (ret < 0) {
>  		if (ret == -ENOMEM || ret == -ENOSPC) {
>  			/*
> -- 
> 2.29.2
>