[dm-devel] [RFC PATCH V2 3/3] dm: support bio polling
Ming Lei
ming.lei at redhat.com
Fri Jun 18 13:29:47 UTC 2021
Hello Jeffle,
On Fri, Jun 18, 2021 at 04:19:10PM +0800, JeffleXu wrote:
>
>
> On 6/17/21 6:35 PM, Ming Lei wrote:
> > Support bio(REQ_POLLED) polling in the following approach:
> >
> > 1) only support io polling on normal READ/WRITE, and other abnormal IOs
> > still fallback on IRQ mode, so the target io is exactly inside the dm
> > io.
> >
> > 2) hold one refcnt on io->io_count after submitting this dm bio with
> > REQ_POLLED
> >
> > 3) support dm native bio splitting, any dm io instance associated with
> > current bio will be added into one list which head is bio->bi_end_io
> > which will be recovered before ending this bio
> >
> > 4) implement .poll_bio() callback, call bio_poll() on the single target
> > bio inside the dm io which is retrieved via bio->bi_bio_drv_data; call
> > dec_pending() after the target io is done in .poll_bio()
> >
> > 4) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
> > which is based on Jeffle's previous patch.
> >
> > Signed-off-by: Ming Lei <ming.lei at redhat.com>
> > ---
> > drivers/md/dm-table.c | 24 +++++++++
> > drivers/md/dm.c | 111 ++++++++++++++++++++++++++++++++++++++++--
> > 2 files changed, 132 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > index ee47a332b462..b14b379442d2 100644
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -1491,6 +1491,12 @@ struct dm_target *dm_table_find_target(struct dm_table *t, sector_t sector)
> > return &t->targets[(KEYS_PER_NODE * n) + k];
> > }
> >
> > +static int device_not_poll_capable(struct dm_target *ti, struct dm_dev *dev,
> > + sector_t start, sector_t len, void *data)
> > +{
> > + return !blk_queue_poll(bdev_get_queue(dev->bdev));
> > +}
> > +
> > /*
> > * type->iterate_devices() should be called when the sanity check needs to
> > * iterate and check all underlying data devices. iterate_devices() will
> > @@ -1541,6 +1547,11 @@ static int count_device(struct dm_target *ti, struct dm_dev *dev,
> > return 0;
> > }
> >
> > +static int dm_table_supports_poll(struct dm_table *t)
> > +{
> > + return !dm_table_any_dev_attr(t, device_not_poll_capable, NULL);
> > +}
> > +
> > /*
> > * Check whether a table has no data devices attached using each
> > * target's iterate_devices method.
> > @@ -2078,6 +2089,19 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
> >
> > dm_update_keyslot_manager(q, t);
> > blk_queue_update_readahead(q);
> > +
> > + /*
> > + * Check for request-based device is remained to
> > + * dm_mq_init_request_queue()->blk_mq_init_allocated_queue().
> > + * For bio-based device, only set QUEUE_FLAG_POLL when all underlying
> > + * devices supporting polling.
> > + */
> > + if (__table_type_bio_based(t->type)) {
> > + if (dm_table_supports_poll(t))
> > + blk_queue_flag_set(QUEUE_FLAG_POLL, q);
> > + else
> > + blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
> > + }
> > }
> >
> > unsigned int dm_table_get_num_targets(struct dm_table *t)
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index 363f12a285ce..9745c3deacc3 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -39,6 +39,8 @@
> > #define DM_COOKIE_ENV_VAR_NAME "DM_COOKIE"
> > #define DM_COOKIE_LENGTH 24
> >
> > +#define REQ_SAVED_END_IO REQ_DRV
> > +
> > static const char *_name = DM_NAME;
> >
> > static unsigned int major = 0;
> > @@ -97,8 +99,11 @@ struct dm_io {
> > unsigned magic;
> > struct mapped_device *md;
> > blk_status_t status;
> > + bool submit_as_polled;
> > atomic_t io_count;
> > struct bio *orig_bio;
> > + void *saved_bio_end_io;
> > + struct hlist_node node;
> > unsigned long start_time;
> > spinlock_t endio_lock;
> > struct dm_stats_aux stats_aux;
> > @@ -687,6 +692,8 @@ static struct dm_target_io *alloc_tio(struct clone_info *ci, struct dm_target *t
> > tio->ti = ti;
> > tio->target_bio_nr = target_bio_nr;
> >
> > + WARN_ON_ONCE(ci->io->submit_as_polled && !tio->inside_dm_io);
> > +
> > return tio;
> > }
> >
> > @@ -938,8 +945,12 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
> > end_io_acct(io);
> > free_io(md, io);
> >
> > - if (io_error == BLK_STS_DM_REQUEUE)
> > + if (io_error == BLK_STS_DM_REQUEUE) {
> > + /* not poll any more in case of requeue */
> > + if (bio->bi_opf & REQ_POLLED)
> > + bio->bi_opf &= ~REQ_POLLED;
> > return;
> > + }
>
> Actually I'm not sure why REQ_POLLED should be cleared here.
It this is one dm native split bio(the one returned from bio_split() in
__split_and_process_bio()), we will never get chance to poll it after it
is requeued.
>
> Besides, there's one corner case where bio is submitted when dm device
> is suspended.
>
> ```
> static blk_qc_t dm_submit_bio(struct bio *bio)
>
> /* If suspended, queue this IO for later */
> if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
> if (bio->bi_opf & REQ_NOWAIT)
> bio_wouldblock_error(bio);
> else if (bio->bi_opf & REQ_RAHEAD)
> bio_io_error(bio);
> else
> queue_io(md, bio);
> goto out;
> }
> ```
>
> When the dm device is suspended with DMF_BLOCK_IO_FOR_SUSPEND, the
> submitted bio is inserted into @md->deferred list by calling queue_io().
> Then the following bio_poll() calling will trigger the
> 'WARN_ON_ONCE(!saved_bi_end_io)' in dm_poll_bio(), if the previously
> submitted bio is still buffered in @md->deferred list.
Good catch, bio_poll() shouldn't call on bio which isn't submitted
really, we can clear REQ_POLLED for avoiding the race.
>
> ```
> submit_bio()
> dm_submit_bio
> queue_io
>
> bio_poll
> dm_poll_bio
> ```
>
>
> >
> > if ((bio->bi_opf & REQ_PREFLUSH) && bio->bi_iter.bi_size) {
> > /*
> > @@ -1590,6 +1601,12 @@ static int __split_and_process_non_flush(struct clone_info *ci)
> > if (__process_abnormal_io(ci, ti, &r))
> > return r;
> >
> > + /*
> > + * Only support bio polling for normal IO, and the target io is
> > + * exactly inside the dm io instance
> > + */
> > + ci->io->submit_as_polled = !!(ci->bio->bi_opf & REQ_POLLED);
> > +
> > len = min_t(sector_t, max_io_len(ti, ci->sector), ci->sector_count);
> >
> > r = __clone_and_map_data_bio(ci, ti, ci->sector, &len);
> > @@ -1608,6 +1625,22 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md,
> > ci->map = map;
> > ci->io = alloc_io(md, bio);
> > ci->sector = bio->bi_iter.bi_sector;
> > +
> > + if (bio->bi_opf & REQ_POLLED) {
> > + INIT_HLIST_NODE(&ci->io->node);
> > +
> > + /*
> > + * Save .bi_end_io into dm_io, so that we can reuse .bi_end_io
> > + * for storing dm_io list
> > + */
> > + if (bio->bi_opf & REQ_SAVED_END_IO) {
> > + ci->io->saved_bio_end_io = NULL;
> > + } else {
> > + ci->io->saved_bio_end_io = bio->bi_end_io;
> > + INIT_HLIST_HEAD((struct hlist_head *)&bio->bi_end_io);
> > + bio->bi_opf |= REQ_SAVED_END_IO;
> > + }
> > + }
> > }
> >
> > #define __dm_part_stat_sub(part, field, subnd) \
> > @@ -1666,8 +1699,19 @@ static void __split_and_process_bio(struct mapped_device *md,
> > }
> > }
> >
> > - /* drop the extra reference count */
> > - dec_pending(ci.io, errno_to_blk_status(error));
> > + /*
> > + * Drop the extra reference count for non-POLLED bio, and hold one
> > + * reference for POLLED bio, which will be released in dm_poll_bio
> > + *
> > + * Add every dm_io instance into the hlist_head which is stored in
> > + * bio->bi_end_io, so that dm_poll_bio can poll them all.
> > + */
> > + if (!ci.io->submit_as_polled)
> > + dec_pending(ci.io, errno_to_blk_status(error));
> > + else
> > + hlist_add_head(&ci.io->node,
> > + (struct hlist_head *)&bio->bi_end_io);
> > +
> > }
> >
> > static void dm_submit_bio(struct bio *bio)
> > @@ -1707,6 +1751,66 @@ static void dm_submit_bio(struct bio *bio)
> > dm_put_live_table(md, srcu_idx);
> > }
> >
> > +static int dm_poll_dm_io(struct dm_io *io, unsigned int flags)
> > +{
> > + if (!io || !io->submit_as_polled)
> > + return 0;
>
> Wondering if '!io->submit_as_polled' is necessary here. dm_io will be
> added into the hash list only when 'io->submit_as_polled' is true in
> __split_and_process_bio.
OK, we can kill the above check.
>
>
> > +
> > + WARN_ON_ONCE(!io->tio.inside_dm_io);
> > + bio_poll(&io->tio.clone, flags);
> > +
> > + /* bio_poll holds the last reference */
> > + if (atomic_read(&io->io_count) == 1)
> > + return 1;
> > + return 0;
> > +}
> > +
> > +static int dm_poll_bio(struct bio *bio, unsigned int flags)
> > +{
> > + struct dm_io *io;
> > + void *saved_bi_end_io = NULL;
> > + struct hlist_head tmp = HLIST_HEAD_INIT;
> > + struct hlist_head *head = (struct hlist_head *)&bio->bi_end_io;
> > + struct hlist_node *next;
> > +
> > + /*
> > + * This bio can be submitted from FS as POLLED so that FS may keep
> > + * polling even though the flag is cleared by bio splitting or
> > + * requeue, so return immediately.
> > + */
> > + if (!(bio->bi_opf & REQ_POLLED))
> > + return 0;
> > +
> > + hlist_move_list(head, &tmp);
> > +
> > + hlist_for_each_entry(io, &tmp, node) {
> > + if (io->saved_bio_end_io && !saved_bi_end_io) {
>
> Seems that checking if saved_bi_end_io NULL here is not necessary, since
> you will exit thte loop once saved_bi_end_io is assigned.
Yeah, indeed.
>
>
>
> > + saved_bi_end_io = io->saved_bio_end_io;
> > + break;
> > + }
> > + }
> > +
> > + /* restore .bi_end_io before completing dm io */
> > + WARN_ON_ONCE(!saved_bi_end_io);
>
> Abnormal bio flagged with REQ_POLLED (is this possible?) can still be
> called with dm_poll_bio(), and will trigger this warning.
OK, I think we need to return 0 from dm_poll_bio() immediately if
the hlist stored in bio->bi_end_io is empty.
Thanks,
Ming
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