Re: [PATCH 12/15] block: switch polling to be bio based

Sagi Grimberg
2021-05-12T22:58:26Z

On 5/12/21 6:15 AM, Christoph Hellwig wrote:
> Replace the blk_poll interface that requires the caller to keep a queue
> and cookie from the submissions with polling based on the bio.
>
> Polling for the bio itself leads to a few advantages:
>
> - the cookie construction can made entirely private in blk-mq.c
> - the caller does not need to remember the request_queue and cookie
> separately and thus sidesteps their lifetime issues
> - keeping the device and the cookie inside the bio allows to trivially
> support polling BIOs remapping by stacking drivers
> - a lot of code to propagate the cookie back up the submission path can
> be removed entirely.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> arch/m68k/emu/nfblock.c | 3 +-
> arch/xtensa/platforms/iss/simdisk.c | 3 +-
> block/bio.c | 1 +
> block/blk-core.c | 120 +++++++++++++++++++++-------
> block/blk-mq.c | 73 ++++++-----------
> block/blk-mq.h | 2 +
> drivers/block/brd.c | 12 ++-
> drivers/block/drbd/drbd_int.h | 2 +-
> drivers/block/drbd/drbd_req.c | 3 +-
> drivers/block/n64cart.c | 12 ++-
> drivers/block/null_blk/main.c | 3 +-
> drivers/block/pktcdvd.c | 7 +-
> drivers/block/ps3vram.c | 6 +-
> drivers/block/rsxx/dev.c | 7 +-
> drivers/block/zram/zram_drv.c | 10 +--
> drivers/lightnvm/pblk-init.c | 6 +-
> drivers/md/bcache/request.c | 13 ++-
> drivers/md/bcache/request.h | 4 +-
> drivers/md/dm.c | 28 +++----
> drivers/md/md.c | 10 +--
> drivers/nvdimm/blk.c | 5 +-
> drivers/nvdimm/btt.c | 5 +-
> drivers/nvdimm/pmem.c | 3 +-
> drivers/nvme/host/core.c | 2 +-
> drivers/nvme/host/multipath.c | 6 +-
> drivers/s390/block/dcssblk.c | 7 +-
> drivers/s390/block/xpram.c | 5 +-
> fs/block_dev.c | 25 ++----
> fs/btrfs/inode.c | 8 +-
> fs/ext4/file.c | 2 +-
> fs/gfs2/file.c | 4 +-
> fs/iomap/direct-io.c | 39 +++------
> fs/xfs/xfs_file.c | 2 +-
> fs/zonefs/super.c | 2 +-
> include/linux/bio.h | 2 +-
> include/linux/blk-mq.h | 15 +---
> include/linux/blk_types.h | 12 ++-
> include/linux/blkdev.h | 8 +-
> include/linux/fs.h | 6 +-
> include/linux/iomap.h | 3 +-
> mm/page_io.c | 8 +-
> 41 files changed, 223 insertions(+), 271 deletions(-)
>
> diff --git a/arch/m68k/emu/nfblock.c b/arch/m68k/emu/nfblock.c
> index ba808543161a..dd36808f0d5e 100644
> --- a/arch/m68k/emu/nfblock.c
> +++ b/arch/m68k/emu/nfblock.c
> @@ -59,7 +59,7 @@ struct nfhd_device {
> struct gendisk *disk;
> };
>
> -static blk_qc_t nfhd_submit_bio(struct bio *bio)
> +static void nfhd_submit_bio(struct bio *bio)
> {
> struct nfhd_device *dev = bio->bi_bdev->bd_disk->private_data;
> struct bio_vec bvec;
> @@ -77,7 +77,6 @@ static blk_qc_t nfhd_submit_bio(struct bio *bio)
> sec += len;
> }
> bio_endio(bio);
> - return BLK_QC_T_NONE;
> }
>
> static int nfhd_getgeo(struct block_device *bdev, struct hd_geometry *geo)
> diff --git a/arch/xtensa/platforms/iss/simdisk.c b/arch/xtensa/platforms/iss/simdisk.c
> index fc09be7b1347..182825d639e2 100644
> --- a/arch/xtensa/platforms/iss/simdisk.c
> +++ b/arch/xtensa/platforms/iss/simdisk.c
> @@ -101,7 +101,7 @@ static void simdisk_transfer(struct simdisk *dev, unsigned long sector,
> spin_unlock(&dev->lock);
> }
>
> -static blk_qc_t simdisk_submit_bio(struct bio *bio)
> +static void simdisk_submit_bio(struct bio *bio)
> {
> struct simdisk *dev = bio->bi_bdev->bd_disk->private_data;
> struct bio_vec bvec;
> @@ -119,7 +119,6 @@ static blk_qc_t simdisk_submit_bio(struct bio *bio)
> }
>
> bio_endio(bio);
> - return BLK_QC_T_NONE;
> }
>
> static int simdisk_open(struct block_device *bdev, fmode_t mode)
> diff --git a/block/bio.c b/block/bio.c
> index de5505d7018e..986908cc99d4 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -250,6 +250,7 @@ void bio_init(struct bio *bio, struct bio_vec *table,
> memset(bio, 0, sizeof(*bio));
> atomic_set(&bio->__bi_remaining, 1);
> atomic_set(&bio->__bi_cnt, 1);
> + bio->bi_cookie = BLK_QC_T_NONE;
>
> bio->bi_io_vec = table;
> bio->bi_max_vecs = max_vecs;
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 94a817532472..c024cba98195 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -910,18 +910,18 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
> return false;
> }
>
> -static blk_qc_t __submit_bio(struct bio *bio)
> +static void __submit_bio(struct bio *bio)
> {
> struct gendisk *disk = bio->bi_bdev->bd_disk;
> - blk_qc_t ret = BLK_QC_T_NONE;
>
> if (blk_crypto_bio_prep(&bio)) {
> - if (!disk->fops->submit_bio)
> - return blk_mq_submit_bio(bio);
> - ret = disk->fops->submit_bio(bio);
> + if (!disk->fops->submit_bio) {
> + blk_mq_submit_bio(bio);
> + return;
> + }
> + disk->fops->submit_bio(bio);
> }
> blk_queue_exit(disk->queue);
> - return ret;
> }
>
> /*
> @@ -943,10 +943,9 @@ static blk_qc_t __submit_bio(struct bio *bio)
> * bio_list_on_stack[1] contains bios that were submitted before the current
> * ->submit_bio_bio, but that haven't been processed yet.
> */
> -static blk_qc_t __submit_bio_noacct(struct bio *bio)
> +static void __submit_bio_noacct(struct bio *bio)
> {
> struct bio_list bio_list_on_stack[2];
> - blk_qc_t ret = BLK_QC_T_NONE;
>
> BUG_ON(bio->bi_next);
>
> @@ -966,7 +965,7 @@ 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);
> + __submit_bio(bio);
>
> /*
> * Sort new bios into those for a lower level and those for the
> @@ -989,13 +988,11 @@ 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;
> }
>
> -static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
> +static void __submit_bio_noacct_mq(struct bio *bio)
> {
> struct bio_list bio_list[2] = { };
> - blk_qc_t ret = BLK_QC_T_NONE;
>
> current->bio_list = bio_list;
>
> @@ -1007,15 +1004,13 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
>
> if (!blk_crypto_bio_prep(&bio)) {
> blk_queue_exit(disk->queue);
> - ret = BLK_QC_T_NONE;
> continue;
> }
>
> - ret = blk_mq_submit_bio(bio);
> + blk_mq_submit_bio(bio);
> } while ((bio = bio_list_pop(&bio_list[0])));
>
> current->bio_list = NULL;
> - return ret;
> }
>
> /**
> @@ -1027,10 +1022,10 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
> * systems and other upper level users of the block layer should use
> * submit_bio() instead.
> */
> -blk_qc_t submit_bio_noacct(struct bio *bio)
> +void submit_bio_noacct(struct bio *bio)
> {
> if (!submit_bio_checks(bio))
> - return BLK_QC_T_NONE;
> + return;
>
> /*
> * We only want one ->submit_bio to be active at a time, else stack
> @@ -1038,14 +1033,12 @@ blk_qc_t submit_bio_noacct(struct bio *bio)
> * to collect a list of requests submited by a ->submit_bio method while
> * it is active, and then process them after it returned.
> */
> - if (current->bio_list) {
> + if (current->bio_list)
> bio_list_add(&current->bio_list[0], bio);
> - return BLK_QC_T_NONE;
> - }
> -
> - if (!bio->bi_bdev->bd_disk->fops->submit_bio)
> - return __submit_bio_noacct_mq(bio);
> - return __submit_bio_noacct(bio);
> + else if (!bio->bi_bdev->bd_disk->fops->submit_bio)
> + __submit_bio_noacct_mq(bio);
> + else
> + __submit_bio_noacct(bio);
> }
> EXPORT_SYMBOL(submit_bio_noacct);
>
> @@ -1062,10 +1055,10 @@ EXPORT_SYMBOL(submit_bio_noacct);
> * in @bio. The bio must NOT be touched by thecaller until ->bi_end_io() has
> * been called.
> */
> -blk_qc_t submit_bio(struct bio *bio)
> +void submit_bio(struct bio *bio)
> {
> if (blkcg_punt_bio_submit(bio))
> - return BLK_QC_T_NONE;
> + return;
>
> /*
> * If it's a regular read/write or a barrier with data attached,
> @@ -1097,19 +1090,84 @@ blk_qc_t submit_bio(struct bio *bio)
> if (unlikely(bio_op(bio) == REQ_OP_READ &&
> bio_flagged(bio, BIO_WORKINGSET))) {
> unsigned long pflags;
> - blk_qc_t ret;
>
> psi_memstall_enter(&pflags);
> - ret = submit_bio_noacct(bio);
> + submit_bio_noacct(bio);
> psi_memstall_leave(&pflags);
> -
> - return ret;
> + return;
> }
>
> - return submit_bio_noacct(bio);
> + submit_bio_noacct(bio);
> }
> EXPORT_SYMBOL(submit_bio);
>
> +/**
> + * bio_poll - poll for BIO completions
> + * @bio: bio to poll for
> + * @flags: BLK_POLL_* flags that control the behavior
> + *
> + * Poll for completions on queue associated with the bio. Returns number of
> + * completed entries found.
> + *
> + * Note: the caller must either be the context that submitted @bio, or
> + * be in a RCU critical section to prevent freeing of @bio.
> + */
> +int bio_poll(struct bio *bio, unsigned int flags)
> +{
> + struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> + blk_qc_t cookie = READ_ONCE(bio->bi_cookie);
> +
> + if (cookie == BLK_QC_T_NONE ||
> + !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> + return 0;
> +
> + if (current->plug)
> + blk_flush_plug_list(current->plug, false);
> +
> + /* not yet implemented, so this should not happen */
> + if (WARN_ON_ONCE(!queue_is_mq(q)))

What happens if the I/O wasn't (yet) queued to the bottom device
(i.e. no available path in nvme-mpath)?

In this case the disk would be the mpath device node (which is
not mq...)