Re: [PATCH 03/16] block: add bio_set_polled() helper

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jan 08, 2019 at 09:56:32AM -0700, Jens Axboe wrote:
> For the upcoming async polled IO, we can't sleep allocating requests.
> If we do, then we introduce a deadlock where the submitter already
> has async polled IO in-flight, but can't wait for them to complete
> since polled requests must be active found and reaped.
> 
> Utilize the helper in the blockdev DIRECT_IO code.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/block_dev.c      |  4 ++--
>  include/linux/bio.h | 14 ++++++++++++++
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 5415579f3e14..2ebd2a0d7789 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -233,7 +233,7 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
>  		task_io_account_write(ret);
>  	}
>  	if (iocb->ki_flags & IOCB_HIPRI)
> -		bio.bi_opf |= REQ_HIPRI;
> +		bio_set_polled(&bio, iocb);
>  
>  	qc = submit_bio(&bio);
>  	for (;;) {
> @@ -401,7 +401,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  		nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
>  		if (!nr_pages) {
>  			if (iocb->ki_flags & IOCB_HIPRI)
> -				bio->bi_opf |= REQ_HIPRI;
> +				bio_set_polled(bio, iocb);
>  
>  			qc = submit_bio(bio);
>  			WRITE_ONCE(iocb->ki_cookie, qc);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 7380b094dcca..f6f0a2b3cbc8 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -823,5 +823,19 @@ static inline int bio_integrity_add_page(struct bio *bio, struct page *page,
>  
>  #endif /* CONFIG_BLK_DEV_INTEGRITY */
>  
> +/*
> + * Mark a bio as polled. Note that for async polled IO, the caller must
> + * expect -EWOULDBLOCK if we cannot allocate a request (or other resources).
> + * We cannot block waiting for requests on polled IO, as those completions
> + * must be found by the caller. This is different than IRQ driven IO, where
> + * it's safe to wait for IO to complete.
> + */
> +static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
> +{
> +	bio->bi_opf |= REQ_HIPRI;
> +	if (!is_sync_kiocb(kiocb))
> +		bio->bi_opf |= REQ_NOWAIT;
> +}
> +

REQ_NOWAIT doesn't cover allocating split bio, is that a issue?

BTW, could you explain a bit about the deadlock in case of sleep from
request allocation?

Thanks,
Ming

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