Re: [PATCH 02/15] block: don't try to poll multi-bio I/Os in __blkdev_direct_IO

[Kanchan Joshi <joshiiitr@gmail.com>]

On Wed, May 12, 2021 at 6:50 PM Christoph Hellwig <hch@lst.de> wrote:
>
> If an iocb is split into multiple bios we can't poll for both.  So don't
> bother to even try to poll in that case.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/block_dev.c | 37 ++++++++++++++-----------------------
>  1 file changed, 14 insertions(+), 23 deletions(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index b8abccd03e5d..0080a3b710b4 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -375,7 +375,7 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>         struct blk_plug plug;
>         struct blkdev_dio *dio;
>         struct bio *bio;
> -       bool is_poll = (iocb->ki_flags & IOCB_HIPRI) != 0;
> +       bool is_poll = (iocb->ki_flags & IOCB_HIPRI), do_poll = false;
>         bool is_read = (iov_iter_rw(iter) == READ), is_sync;
>         loff_t pos = iocb->ki_pos;
>         blk_qc_t qc = BLK_QC_T_NONE;
> @@ -437,22 +437,9 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>                 pos += bio->bi_iter.bi_size;
>
>                 nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS);
> -               if (!nr_pages) {
> -                       bool polled = false;
> -
> -                       if (iocb->ki_flags & IOCB_HIPRI) {
> -                               bio_set_polled(bio, iocb);
> -                               polled = true;
> -                       }
> -
> -                       qc = submit_bio(bio);
> -
> -                       if (polled)
> -                               WRITE_ONCE(iocb->ki_cookie, qc);
> -                       break;
> -               }
> -
> -               if (!dio->multi_bio) {
> +               if (dio->multi_bio) {
> +                       atomic_inc(&dio->ref);
> +               } else if (nr_pages) {
>                         /*
>                          * AIO needs an extra reference to ensure the dio
>                          * structure which is embedded into the first bio
> @@ -462,11 +449,16 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>                                 bio_get(bio);
>                         dio->multi_bio = true;
>                         atomic_set(&dio->ref, 2);
> -               } else {
> -                       atomic_inc(&dio->ref);
> +               } else if (is_poll) {
> +                       bio_set_polled(bio, iocb);
> +                       do_poll = true;
> +               }
> +               qc = submit_bio(bio);
> +               if (!nr_pages) {
> +                       if (do_poll)
> +                               WRITE_ONCE(iocb->ki_cookie, qc);
> +                       break;
>                 }
> -
> -               submit_bio(bio);
>                 bio = bio_alloc(GFP_KERNEL, nr_pages);
>         }

dio->ref update goes amiss here.
For multi-bio, if the original code sets it as N, this will set it as
N+1, causing endless-wait for the caller.

-- 
Kanchan