Re: [RFC PATCH 07/13] fs/userfaultfd: support read_iter to use io_uring

[Jens Axboe]

On 11/28/20 5:45 PM, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> iouring with userfaultfd cannot currently be used fixed buffers since
> userfaultfd does not provide read_iter(). This is required to allow
> asynchronous (queued) reads from userfaultfd.
> 
> To support async-reads of userfaultfd provide read_iter() instead of
> read().
> 
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: io-uring@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
>  fs/userfaultfd.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index b6a04e526025..6333b4632742 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1195,9 +1195,9 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
>  	return ret;
>  }
>  
> -static ssize_t userfaultfd_read(struct file *file, char __user *buf,
> -				size_t count, loff_t *ppos)
> +static ssize_t userfaultfd_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  {
> +	struct file *file = iocb->ki_filp;
>  	struct userfaultfd_ctx *ctx = file->private_data;
>  	ssize_t _ret, ret = 0;
>  	struct uffd_msg msg;
> @@ -1207,16 +1207,18 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	for (;;) {
> -		if (count < sizeof(msg))
> +		if (iov_iter_count(to) < sizeof(msg))
>  			return ret ? ret : -EINVAL;
>  		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);

'no_wait' should be changed to factor in iocb->ki_flags & IOCB_NOWAIT as well,
not just f_flags & O_NONBLOCK.

I didn't check your write_iter, but if appropriate, that should do that
too.

-- 
Jens Axboe