[PATCH RFC v4 1/3] block: add BIO_COMPLETE_IN_TASK for task-context completion

[Tal Zussman <tz2294@columbia.edu>]

Some bio completion handlers need to run in task context but bio_endio()
can be called from IRQ context (e.g. buffer_head writeback). Add a
BIO_COMPLETE_IN_TASK flag that bio submitters can set to request
task-context completion of their bi_end_io callback.

When bio_endio() sees this flag and is running in non-task context, it
queues the bio to a per-cpu list and schedules a work item to call
bi_end_io() from task context. A CPU hotplug dead callback drains any
remaining bios from the departing CPU's batch.

This will be used to enable RWF_DONTCACHE for block devices, and could
be used for other subsystems like fscrypt that need task-context bio
completion.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 block/bio.c               | 84 ++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/blk_types.h |  1 +
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 8203bb7455a9..69ee0d93041f 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -18,6 +18,7 @@
 #include <linux/highmem.h>
 #include <linux/blk-crypto.h>
 #include <linux/xarray.h>
+#include <linux/local_lock.h>
 
 #include <trace/events/block.h>
 #include "blk.h"
@@ -1714,6 +1715,60 @@ void bio_check_pages_dirty(struct bio *bio)
 }
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
+struct bio_complete_batch {
+	local_lock_t lock;
+	struct bio_list list;
+	struct work_struct work;
+};
+
+static DEFINE_PER_CPU(struct bio_complete_batch, bio_complete_batch) = {
+	.lock = INIT_LOCAL_LOCK(lock),
+};
+
+static void bio_complete_work_fn(struct work_struct *w)
+{
+	struct bio_complete_batch *batch;
+	struct bio_list list;
+
+again:
+	local_lock_irq(&bio_complete_batch.lock);
+	batch = this_cpu_ptr(&bio_complete_batch);
+	list = batch->list;
+	bio_list_init(&batch->list);
+	local_unlock_irq(&bio_complete_batch.lock);
+
+	while (!bio_list_empty(&list)) {
+		struct bio *bio = bio_list_pop(&list);
+		bio->bi_end_io(bio);
+	}
+
+	local_lock_irq(&bio_complete_batch.lock);
+	batch = this_cpu_ptr(&bio_complete_batch);
+	if (!bio_list_empty(&batch->list)) {
+		local_unlock_irq(&bio_complete_batch.lock);
+
+		if (!need_resched())
+			goto again;
+
+		schedule_work_on(smp_processor_id(), &batch->work);
+		return;
+	}
+	local_unlock_irq(&bio_complete_batch.lock);
+}
+
+static void bio_queue_completion(struct bio *bio)
+{
+	struct bio_complete_batch *batch;
+	unsigned long flags;
+
+	local_lock_irqsave(&bio_complete_batch.lock, flags);
+	batch = this_cpu_ptr(&bio_complete_batch);
+	bio_list_add(&batch->list, bio);
+	local_unlock_irqrestore(&bio_complete_batch.lock, flags);
+
+	schedule_work_on(smp_processor_id(), &batch->work);
+}
+
 static inline bool bio_remaining_done(struct bio *bio)
 {
 	/*
@@ -1788,7 +1843,9 @@ void bio_endio(struct bio *bio)
 	}
 #endif
 
-	if (bio->bi_end_io)
+	if (!in_task() && bio_flagged(bio, BIO_COMPLETE_IN_TASK))
+		bio_queue_completion(bio);
+	else if (bio->bi_end_io)
 		bio->bi_end_io(bio);
 }
 EXPORT_SYMBOL(bio_endio);
@@ -1974,6 +2031,21 @@ int bioset_init(struct bio_set *bs,
 }
 EXPORT_SYMBOL(bioset_init);
 
+/*
+ * Drain a dead CPU's deferred bio completions. The CPU is dead so no locking
+ * is needed -- no new bios will be queued to it.
+ */
+static int bio_complete_batch_cpu_dead(unsigned int cpu)
+{
+	struct bio_complete_batch *batch = per_cpu_ptr(&bio_complete_batch, cpu);
+	struct bio *bio;
+
+	while ((bio = bio_list_pop(&batch->list)))
+		bio->bi_end_io(bio);
+
+	return 0;
+}
+
 static int __init init_bio(void)
 {
 	int i;
@@ -1988,6 +2060,16 @@ static int __init init_bio(void)
 				SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
 	}
 
+	for_each_possible_cpu(i) {
+		struct bio_complete_batch *batch =
+			per_cpu_ptr(&bio_complete_batch, i);
+
+		bio_list_init(&batch->list);
+		INIT_WORK(&batch->work, bio_complete_work_fn);
+	}
+
+	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "block/bio:complete:dead",
+				NULL, bio_complete_batch_cpu_dead);
 	cpuhp_setup_state_multi(CPUHP_BIO_DEAD, "block/bio:dead", NULL,
 					bio_cpu_dead);
 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 8808ee76e73c..d49d97a050d0 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -322,6 +322,7 @@ enum {
 	BIO_REMAPPED,
 	BIO_ZONE_WRITE_PLUGGING, /* bio handled through zone write plugging */
 	BIO_EMULATES_ZONE_APPEND, /* bio emulates a zone append operation */
+	BIO_COMPLETE_IN_TASK, /* complete bi_end_io() in task context */
 	BIO_FLAG_LAST
 };
 

-- 
2.39.5