Previously, if we ever try to allocate more than once from the same bio
set while running under generic_make_request() (i.e. a stacking block
driver), we risk deadlock.

This is because of the code in generic_make_request() that converts
recursion to iteration; any bios we submit won't actually be submitted
(so they can complete and eventually be freed) until after we return -
this means if we allocate a second bio, we're blocking the first one
from ever being freed.

Thus if enough threads call into a stacking block driver at the same
time with bios that need multiple splits, and the bio_set's reserve gets
used up, we deadlock.

This can be worked around in the driver code - we could check if we're
running under generic_make_request(), then mask out __GFP_WAIT when we
go to allocate a bio, and if the allocation fails punt to workqueue and
retry the allocation.

But this is tricky and not a generic solution. This patch solves it for
all users by inverting the previously described technique. We allocate a
rescuer workqueue for each bio_set, and then in the allocation code if
there are bios on current->bio_list we would be blocking, we punt them
to the rescuer workqueue to be submitted.

Tested it by forcing the rescue codepath to be taken (by disabling the
first GFP_NOWAIT) attempt, and then ran it with bcache (which does a lot
of arbitrary bio splitting) and verified that the rescuer was being
invoked.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
---
fs/bio.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++ +--
include/linux/bio.h | 9 ++++++
2 files changed, 93 insertions(+), 3 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index 13e9567..244007f 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -295,6 +295,43 @@ void bio_reset(struct bio *bio)
}
EXPORT_SYMBOL(bio_reset);

+static void bio_alloc_rescue(struct work_struct *work)
+{
+ struct bio_set *bs = container_of(work, struct bio_set, rescue_work);
+ struct bio *bio;
+
+ while (1) {
+ spin_lock(&bs->rescue_lock);
+ bio = bio_list_pop(&bs->rescue_list);
+ spin_unlock(&bs->rescue_lock);
+
+ if (!bio)
+ break;
+
+ generic_make_request(bio);
+ }
+}
+
+static void punt_bios_to_rescuer(struct bio_set *bs)
+{
+ struct bio_list punt, nopunt;
+ struct bio *bio;
+
+ bio_list_init(&punt);
+ bio_list_init(&nopunt);
+
+ while ((bio = bio_list_pop(current->bio_list)))
+ bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
+
+ *current->bio_list = nopunt;
+
+ spin_lock(&bs->rescue_lock);
+ bio_list_merge(&bs->rescue_list, &punt);
+ spin_unlock(&bs->rescue_lock);
+
+ queue_work(bs->rescue_workqueue, &bs->rescue_work);
+}
+
/**
* bio_alloc_bioset - allocate a bio for I/O
* @gfp_mask: the GFP_ mask given to the slab allocator
@@ -317,6 +354,7 @@ EXPORT_SYMBOL(bio_reset);
*/
struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
{
+ gfp_t saved_gfp = gfp_mask;
unsigned front_pad;
unsigned inline_vecs;
unsigned long idx = BIO_POOL_NONE;
@@ -334,13 +372,37 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
front_pad = 0;
inline_vecs = nr_iovecs;
} else {
+ /*
+ * generic_make_request() converts recursion to iteration; this
+ * means if we're running beneath it, any bios we allocate and
+ * submit will not be submitted (and thus freed) until after we
+ * return.
+ *
+ * This exposes us to a potential deadlock if we allocate
+ * multiple bios from the same bio_set() while running
+ * underneath generic_make_request(). If we were to allocate
+ * multiple bios (say a stacking block driver that was splitting
+ * bios), we would deadlock if we exhausted the mempool's
+ * reserve.
+ *
+ * We solve this, and guarantee forward progress, with a rescuer
+ * workqueue per bio_set. If we go to allocate and there are
+ * bios on current->bio_list, we first try the allocation
+ * without __GFP_WAIT; if that fails, we punt those bios we
+ * would be blocking to the rescuer workqueue before we retry
+ * with the original gfp_flags.
+ */
+
+ if (current->bio_list && !bio_list_empty(current->bio_list))
+ gfp_mask &= ~__GFP_WAIT;
+retry:
p = mempool_alloc(bs->bio_pool, gfp_mask);
front_pad = bs->front_pad;
inline_vecs = BIO_INLINE_VECS;
}

if (unlikely(!p))
- return NULL;
+ goto err;

bio = p + front_pad;
bio_init(bio);
@@ -361,6 +423,13 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)

err_free:
mempool_free(p, bs->bio_pool);
+err:
+ if (gfp_mask != saved_gfp) {
+ punt_bios_to_rescuer(bs);
+ gfp_mask = saved_gfp;
+ goto retry;
+ }
+
return NULL;
}
EXPORT_SYMBOL(bio_alloc_bioset);
@@ -1570,6 +1639,9 @@ static void biovec_free_pools(struct bio_set *bs)

void bioset_free(struct bio_set *bs)
{
+ if (bs->rescue_workqueue)
+ destroy_workqueue(bs->rescue_workqueue);
+
if (bs->bio_pool)
mempool_destroy(bs->bio_pool);

@@ -1605,6 +1677,10 @@ struct bio_set *bioset_create(unsigned int pool_size, unsigned int front_pad)

bs->front_pad = front_pad;

+ spin_lock_init(&bs->rescue_lock);
+ bio_list_init(&bs->rescue_list);
+ INIT_WORK(&bs->rescue_work, bio_alloc_rescue);
+
bs->bio_slab = bio_find_or_create_slab(front_pad + back_pad);
if (!bs->bio_slab) {
kfree(bs);
@@ -1615,9 +1691,14 @@ struct bio_set *bioset_create(unsigned int pool_size, unsigned int front_pad)
if (!bs->bio_pool)
goto bad;

- if (!biovec_create_pools(bs, pool_size))
- return bs;
+ if (biovec_create_pools(bs, pool_size))
+ goto bad;
+
+ bs->rescue_workqueue = alloc_workqueue("bioset", WQ_MEM_RECLAIM, 0);
+ if (!bs->rescue_workqueue)
+ goto bad;

+ return bs;
bad:
bioset_free(bs);
return NULL;
diff --git a/include/linux/bio.h b/include/linux/bio.h
index a2bfe3a..c32ea0d 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -491,6 +491,15 @@ struct bio_set {
mempool_t *bio_integrity_pool;
#endif
mempool_t *bvec_pool;
+
+ /*
+ * Deadlock avoidance for stacking block drivers: see comments in
+ * bio_alloc_bioset() for details
+ */
+ spinlock_t rescue_lock;
+ struct bio_list rescue_list;
+ struct work_struct rescue_work;
+ struct workqueue_struct *rescue_workqueue;
};

struct biovec_slab {
--
1.7.12

--
dm-devel mailing list
dm-devel@redhat.com
https://www.redhat.com/mailman/listinfo/dm-devel