On Mon, 8 Feb 2021, Jeffle Xu wrote:
> Offer one fastpath of bio-based polling when bio submitted to dm device
> is not split.
>
> In this case, there will be only one bio submitted to only one polling
> hw queue of one underlying mq device, and thus we don't need to track
> all split bios or iterate through all polling hw queues. The pointer to
> the polling hw queue the bio submitted to is returned here as the
> returned cookie.
This doesn't seem safe - note that between submit_bio() and blk_poll(), no
locks are held - so the device mapper device may be reconfigured
arbitrarily. When you call blk_poll() with a pointer returned by
submit_bio(), the pointer may point to a stale address.
Mikulas
> In this case, the polling routine will call
> mq_ops->poll() directly with the hw queue converted from the input
> cookie.
>
> If the original bio submitted to dm device is split to multiple bios and
> thus submitted to multiple polling hw queues, the bio submission routine
> will return BLK_QC_T_BIO_MULTI, while the polling routine will fall
> back to iterating all hw queues (in polling mode) of all underlying mq
> devices.
>
> Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> ---
> block/blk-core.c | 33 +++++++++++++++++++++++++++++++--
> include/linux/blk_types.h | 8 ++++++++
> include/linux/types.h | 2 +-
> 3 files changed, 40 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 37aa513da5f2..cb24b33a4870 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -956,11 +956,19 @@ static blk_qc_t __submit_bio(struct bio *bio)
> * bio_list_on_stack[0] contains bios submitted by the current ->submit_bio.
> * bio_list_on_stack[1] contains bios that were submitted before the current
> * ->submit_bio_bio, but that haven't been processed yet.
> + *
> + * Return:
> + * - BLK_QC_T_NONE, no need for IO polling.
> + * - BLK_QC_T_BIO_MULTI, @bio gets split and enqueued into multi hw queues.
> + * - Otherwise, @bio is not split, returning the pointer to the corresponding
> + * hw queue that the bio enqueued into as the returned cookie.
> */
> static blk_qc_t __submit_bio_noacct(struct bio *bio)
> {
> struct bio_list bio_list_on_stack[2];
> blk_qc_t ret = BLK_QC_T_NONE;
> + struct request_queue *top_q = bio->bi_disk->queue;
> + bool poll_on = test_bit(QUEUE_FLAG_POLL, &top_q->queue_flags);
>
> BUG_ON(bio->bi_next);
>
> @@ -968,6 +976,7 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
> current->bio_list = bio_list_on_stack;
>
> do {
> + blk_qc_t cookie;
> struct request_queue *q = bio->bi_disk->queue;
> struct bio_list lower, same;
>
> @@ -980,7 +989,20 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
> bio_list_on_stack[1] = bio_list_on_stack[0];
> bio_list_init(&bio_list_on_stack[0]);
>
> - ret = __submit_bio(bio);
> + cookie = __submit_bio(bio);
> +
> + if (poll_on &&
> + blk_qc_t_bio_valid(ret) && blk_qc_t_valid(cookie)) {
> + unsigned int queue_num = blk_qc_t_to_queue_num(cookie);
> + struct blk_mq_hw_ctx *hctx = q->queue_hw_ctx[queue_num];
> +
> + cookie = (blk_qc_t)hctx;
> +
> + if (!blk_qc_t_valid(ret)) /* set initial value */
> + ret = cookie;
> + else if (ret != cookie) /* bio got split */
> + ret = BLK_QC_T_BIO_MULTI;
> + }
>
> /*
> * Sort new bios into those for a lower level and those for the
> @@ -1003,6 +1025,7 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
> } while ((bio = bio_list_pop(&bio_list_on_stack[0])));
>
> current->bio_list = NULL;
> +
> return ret;
> }
>
> @@ -1142,7 +1165,13 @@ static int blk_bio_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> do {
> int ret;
>
> - ret = disk->fops->poll(q, cookie);
> + if (blk_qc_t_bio_valid(cookie)) {
> + struct blk_mq_hw_ctx *hctx = (struct blk_mq_hw_ctx *)cookie;
> + struct request_queue *target_q = hctx->queue;
> +
> + ret = blk_mq_poll_hctx(target_q, hctx);
> + } else
> + ret = disk->fops->poll(q, cookie);
> if (ret > 0) {
> __set_current_state(TASK_RUNNING);
> return ret;
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 2e05244fc16d..4173754532c0 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -557,6 +557,14 @@ static inline bool blk_qc_t_is_internal(blk_qc_t cookie)
> return (cookie & BLK_QC_T_INTERNAL) != 0;
> }
>
> +/* Macros for blk_qc_t used for bio-based polling */
> +#define BLK_QC_T_BIO_MULTI -2U
> +
> +static inline bool blk_qc_t_bio_valid(blk_qc_t cookie)
> +{
> + return cookie != BLK_QC_T_BIO_MULTI;
> +}
> +
> struct blk_rq_stat {
> u64 mean;
> u64 min;
> diff --git a/include/linux/types.h b/include/linux/types.h
> index da5ca7e1bea9..f6301014a459 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -126,7 +126,7 @@ typedef u64 sector_t;
> typedef u64 blkcnt_t;
>
> /* cookie used for IO polling */
> -typedef unsigned int blk_qc_t;
> +typedef uintptr_t blk_qc_t;
>
> /*
> * The type of an index into the pagecache.
> --
> 2.27.0
>
> --
> dm-devel mailing list
> dm-devel@xxxxxxxxxx
> https://listman.redhat.com/mailman/listinfo/dm-devel
>
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