block: add ability to flag write back caching on a device
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.
This will replace the (awkward) blk_queue_flush() interface that
drivers currently use to inform the block layer of write cache state
and capabilities.
Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index e5d9148..dce25d8 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -141,6 +141,15 @@
an IO scheduler name to this file will attempt to load that IO scheduler
module, if it isn't already present in the system.
+write_cache (RW)
+----------------
+When read, this file will display whether the device has write back
+caching enabled or not. It will return "write back" for the former
+case, and "write through" for the latter. Writing to this file can
+change the kernels view of the device, but it doesn't alter the
+device state. This means that it might not be safe to toggle the
+setting from "write back" to "write through", since that will also
+eliminate cache flushes issued by the kernel.
Jens Axboe <jens.axboe@oracle.com>, February 2009
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 331e4ee..c903bee 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -846,6 +846,32 @@
}
EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);
+/**
+ * blk_queue_write_cache - configure queue's write cache
+ * @q: the request queue for the device
+ * @wc: write back cache on or off
+ * @fua: device supports FUA writes, if true
+ *
+ * Tell the block layer about the write cache of @q.
+ */
+void blk_queue_write_cache(struct request_queue *q, bool wc, bool fua)
+{
+ spin_lock_irq(q->queue_lock);
+ if (wc) {
+ queue_flag_set(QUEUE_FLAG_WC, q);
+ q->flush_flags = REQ_FLUSH;
+ } else
+ queue_flag_clear(QUEUE_FLAG_WC, q);
+ if (fua) {
+ if (wc)
+ q->flush_flags |= REQ_FUA;
+ queue_flag_set(QUEUE_FLAG_FUA, q);
+ } else
+ queue_flag_clear(QUEUE_FLAG_FUA, 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 995b58d..9920596 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -347,6 +347,38 @@
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 @@
.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 @@
&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 bbaa767..ba72687 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -491,6 +491,8 @@
#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_FUA 24 /* device supports FUA writes */
#define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
(1 << QUEUE_FLAG_STACKABLE) | \
@@ -1009,6 +1011,7 @@
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, bool fua);
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 *);