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

Jens Axboe
2020-11-30T18:21:12Z

On 11/28/20 5:45 PM, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
>
> 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@kernel.dk>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: io-uring@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> 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