[PATCH AUTOSEL for 4.14 08/67] dm: ensure bio submission follows a depth-first tree walk

Sasha Levin
2018-03-08T06:51:20Z

From: NeilBrown <neilb@suse.com>

[ Upstream commit 18a25da84354c6bb655320de6072c00eda6eb602 ]

A dm device can, in general, represent a tree of targets, each of which
handles a sub-range of the range of blocks handled by the parent.

The bio sequencing managed by generic_make_request() requires that bios
are generated and handled in a depth-first manner. Each call to a
make_request_fn() may submit bios to a single member device, and may
submit bios for a reduced region of the same device as the
make_request_fn.

In particular, any bios submitted to member devices must be expected to
be processed in order, so a later one must never wait for an earlier
one.

This ordering is usually achieved by using bio_split() to reduce a bio
to a size that can be completely handled by one target, and resubmitting
the remainder to the originating device. bio_queue_split() shows the
canonical approach.

dm doesn't follow this approach, largely because it has needed to split
bios since long before bio_split() was available. It currently can
submit bios to separate targets within the one dm_make_request() call.
Dependencies between these targets, as can happen with dm-snap, can
cause deadlocks if either bios gets stuck behind the other in the queues
managed by generic_make_request(). This requires the 'rescue'
functionality provided by dm_offload_{start,end}.

Some of this requirement can be removed by changing the order of bio
submission to follow the canonical approach. That is, if dm finds that
it needs to split a bio, the remainder should be sent to
generic_make_request() rather than being handled immediately. This
delays the handling until the first part is completely processed, so the
deadlock problems do not occur.

__split_and_process_bio() can be called both from dm_make_request() and
from dm_wq_work(). When called from dm_wq_work() the current approach
is perfectly satisfactory as each bio will be processed immediately.
When called from dm_make_request(), current->bio_list will be non-NULL,
and in this case it is best to create a separate "clone" bio for the
remainder.

When we use bio_clone_bioset() to split off the front part of a bio
and chain the two together and submit the remainder to
generic_make_request(), it is important that the newly allocated
bio is used as the head to be processed immediately, and the original
bio gets "bio_advance()"d and sent to generic_make_request() as the
remainder. Otherwise, if the newly allocated bio is used as the
remainder, and if it then needs to be split again, then the next
bio_clone_bioset() call will be made while holding a reference a bio
(result of the first clone) from the same bioset. This can potentially
exhaust the bioset mempool and result in a memory allocation deadlock.

Note that there is no race caused by reassigning cio.io->bio after already
calling __map_bio(). This bio will only be dereferenced again after
dec_pending() has found io->io_count to be zero, and this cannot happen
before the dec_pending() call at the end of __split_and_process_bio().

To provide the clone bio when splitting, we use q->bio_split. This
was previously being freed by bio-based dm to avoid having excess
rescuer threads. As bio_split bio sets no longer create rescuer
threads, there is little cost and much gain from restoring the
q->bio_split bio set.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
drivers/md/dm.c | 33 ++++++++++++++++++++++++---------
1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 1dfc855ac708..902b6a5d3a4e 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1497,8 +1497,29 @@ static void __split_and_process_bio(struct mapped_device *md,
} else {
ci.bio = bio;
ci.sector_count = bio_sectors(bio);
- while (ci.sector_count && !error)
+ while (ci.sector_count && !error) {
error = __split_and_process_non_flush(&ci);
+ if (current->bio_list && ci.sector_count && !error) {
+ /*
+ * Remainder must be passed to generic_make_request()
+ * so that it gets handled *after* bios already submitted
+ * have been completely processed.
+ * We take a clone of the original to store in
+ * ci.io->bio to be used by end_io_acct() and
+ * for dec_pending to use for completion handling.
+ * As this path is not used for REQ_OP_ZONE_REPORT,
+ * the usage of io->bio in dm_remap_zone_report()
+ * won't be affected by this reassignment.
+ */
+ struct bio *b = bio_clone_bioset(bio, GFP_NOIO,
+ md->queue->bio_split);
+ ci.io->bio = b;
+ bio_advance(bio, (bio_sectors(bio) - ci.sector_count) << 9);
+ bio_chain(b, bio);
+ generic_make_request(bio);
+ break;
+ }
+ }
}

/* drop the extra reference count */
@@ -1509,8 +1530,8 @@ static void __split_and_process_bio(struct mapped_device *md,
*---------------------------------------------------------------*/

/*
- * The request function that just remaps the bio built up by
- * dm_merge_bvec.
+ * The request function that remaps the bio to one target and
+ * splits off any remainder.
*/
static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
{
@@ -2044,12 +2065,6 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
case DM_TYPE_DAX_BIO_BASED:
dm_init_normal_md_queue(md);
blk_queue_make_request(md->queue, dm_make_request);
- /*
- * DM handles splitting bios as needed. Free the bio_split bioset
- * since it won't be used (saves 1 process per bio-based DM device).
- */
- bioset_free(md->queue->bio_split);
- md->queue->bio_split = NULL;

if (type == DM_TYPE_DAX_BIO_BASED)
queue_flag_set_unlocked(QUEUE_FLAG_DAX, md->queue);
--
2.14.1