[PATCH 5/9] block: add ability to flag write back caching on a device

[Jens Axboe <axboe@fb.com>]

Add an internal helper and flag for setting whether a queue has
write back caching, or write through (or none). Add a sysfs file
to show this as well, and make it changeable from user space.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 block/blk-settings.c   | 11 +++++++++++
 block/blk-sysfs.c      | 39 +++++++++++++++++++++++++++++++++++++++
 include/linux/blkdev.h |  4 ++++
 3 files changed, 54 insertions(+)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index c7bb666aafd1..4dbd511a9889 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -846,6 +846,17 @@ void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
 }
 EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);
 
+void blk_queue_write_cache(struct request_queue *q, bool enabled)
+{
+	spin_lock_irq(q->queue_lock);
+	if (enabled)
+		queue_flag_set(QUEUE_FLAG_WC, q);
+	else
+		queue_flag_clear(QUEUE_FLAG_WC, q);
+	spin_unlock_irq(q->queue_lock);
+}
+EXPORT_SYMBOL_GPL(blk_queue_write_cache);
+
 static int __init blk_settings_init(void)
 {
 	blk_max_low_pfn = max_low_pfn - 1;
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index dd93763057ce..954e510452d7 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -347,6 +347,38 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
 	return ret;
 }
 
+static ssize_t queue_wc_show(struct request_queue *q, char *page)
+{
+	if (test_bit(QUEUE_FLAG_WC, &q->queue_flags))
+		return sprintf(page, "write back\n");
+
+	return sprintf(page, "write through\n");
+}
+
+static ssize_t queue_wc_store(struct request_queue *q, const char *page,
+			      size_t count)
+{
+	int set = -1;
+
+	if (!strncmp(page, "write back", 10))
+		set = 1;
+	else if (!strncmp(page, "write through", 13) ||
+		 !strncmp(page, "none", 4))
+		set = 0;
+
+	if (set == -1)
+		return -EINVAL;
+
+	spin_lock_irq(q->queue_lock);
+	if (set)
+		queue_flag_set(QUEUE_FLAG_WC, q);
+	else
+		queue_flag_clear(QUEUE_FLAG_WC, q);
+	spin_unlock_irq(q->queue_lock);
+
+	return count;
+}
+
 static struct queue_sysfs_entry queue_requests_entry = {
 	.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
 	.show = queue_requests_show,
@@ -478,6 +510,12 @@ static struct queue_sysfs_entry queue_poll_entry = {
 	.store = queue_poll_store,
 };
 
+static struct queue_sysfs_entry queue_wc_entry = {
+	.attr = {.name = "write_cache", .mode = S_IRUGO | S_IWUSR },
+	.show = queue_wc_show,
+	.store = queue_wc_store,
+};
+
 static struct attribute *default_attrs[] = {
 	&queue_requests_entry.attr,
 	&queue_ra_entry.attr,
@@ -503,6 +541,7 @@ static struct attribute *default_attrs[] = {
 	&queue_iostats_entry.attr,
 	&queue_random_entry.attr,
 	&queue_poll_entry.attr,
+	&queue_wc_entry.attr,
 	NULL,
 };
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 7e5d7e018bea..76e875159e52 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -491,15 +491,18 @@ struct request_queue {
 #define QUEUE_FLAG_INIT_DONE   20	/* queue is initialized */
 #define QUEUE_FLAG_NO_SG_MERGE 21	/* don't attempt to merge SG segments*/
 #define QUEUE_FLAG_POLL	       22	/* IO polling enabled if set */
+#define QUEUE_FLAG_WC	       23	/* Write back caching */
 
 #define QUEUE_FLAG_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
 				 (1 << QUEUE_FLAG_STACKABLE)	|	\
 				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
+				 (1 << QUEUE_FLAG_WC)		|	\
 				 (1 << QUEUE_FLAG_ADD_RANDOM))
 
 #define QUEUE_FLAG_MQ_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
 				 (1 << QUEUE_FLAG_STACKABLE)	|	\
 				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
+				 (1 << QUEUE_FLAG_WC)		|	\
 				 (1 << QUEUE_FLAG_POLL))
 
 static inline void queue_lockdep_assert_held(struct request_queue *q)
@@ -1009,6 +1012,7 @@ extern void blk_queue_rq_timed_out(struct request_queue *, rq_timed_out_fn *);
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern void blk_queue_flush(struct request_queue *q, unsigned int flush);
 extern void blk_queue_flush_queueable(struct request_queue *q, bool queueable);
+extern void blk_queue_write_cache(struct request_queue *q, bool enabled);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
 
 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
-- 
2.8.0.rc4.6.g7e4ba36