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 e5d914845be6de4e59d59b7c1e905392d6bc5fb5..dce25d848d92c67074ebd26ffaa526217f8f3957 100644 (file)
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -141,6 +141,15 @@ control of this block device to that new IO scheduler. Note that writing
 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 331e4eee0dda0c29cc673b63c7e7341ae45e5859..c903bee43cf85221a5a007d6c3b60b8b0aa81b23 100644 (file)
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -846,6 +846,32 @@ void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
 }
 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 995b58d46ed109b0c7241b6db5b870ed911dc845..99205965f5596c2c935045c049f5cf9683c7bf42 100644 (file)
--- 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 bbaa767570189537b5ad313d6f0a40c6395cfda7..ba72687c5654184c9267505284871bd648bd462a 100644 (file)
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -491,6 +491,8 @@ 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_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_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, 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 *);