[PATCH 05/14] dm: set the static flush bio device on demand

From: Dennis Zhou
Date: Wed Dec 05 2018 - 12:11:58 EST


The next patch changes the macro bio_set_dev() to associate a bio with a
blkg based on the device set. However, dm creates a static bio to be
used as the basis for cloning empty flush bios on creation. The
bio_set_dev() call in alloc_dev() will cause problems with the next
patch adding association to bio_set_dev() because the call is before the
bdev is associated with a gendisk (bd_disk is %NULL). To get around
this, set the device on the static bio every time and use that to clone
to the other bios.

Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
Acked-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: Alasdair Kergon <agk@xxxxxxxxxx>
---
block/bio.c | 1 +
drivers/md/dm.c | 12 +++++++++++-
2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 452b8e79b998..41ebb3f8e2fc 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2021,6 +2021,7 @@ void bio_disassociate_blkg(struct bio *bio)
bio->bi_blkg = NULL;
}
}
+EXPORT_SYMBOL_GPL(bio_disassociate_blkg);

/**
* __bio_associate_blkg - associate a bio with the a blkg
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index a733e4c920af..ab72d79775ee 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1417,10 +1417,21 @@ static int __send_empty_flush(struct clone_info *ci)
unsigned target_nr = 0;
struct dm_target *ti;

+ /*
+ * Empty flush uses a statically initialized bio, &md->flush_bio, as
+ * the base for cloning. However, blkg association requires that a
+ * bdev is associated with a gendisk, which doesn't happen until the
+ * bdev is opened. So, blkg association is done at issue time of the
+ * flush rather than when the device is created in alloc_dev().
+ */
+ bio_set_dev(ci->bio, ci->io->md->bdev);
+
BUG_ON(bio_has_data(ci->bio));
while ((ti = dm_table_get_target(ci->map, target_nr++)))
__send_duplicate_bios(ci, ti, ti->num_flush_bios, NULL);

+ bio_disassociate_blkg(ci->bio);
+
return 0;
}

@@ -1939,7 +1950,6 @@ static struct mapped_device *alloc_dev(int minor)
goto bad;

bio_init(&md->flush_bio, NULL, 0);
- bio_set_dev(&md->flush_bio, md->bdev);
md->flush_bio.bi_opf = REQ_OP_WRITE | REQ_PREFLUSH | REQ_SYNC;

dm_stats_init(&md->stats);
--
2.17.1