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Commit 93e9d8e8 authored by Jens Axboe's avatar Jens Axboe
Browse files

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: default avatarJens Axboe <axboe@fb.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
parent e0489487

Documentation/block/queue-sysfs.txt

+9 −0
Original line number Diff line number Diff line
@@ -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

block/blk-settings.c

+26 −0
Original line number Diff line number Diff line
@@ -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;

block/blk-sysfs.c

+39 −0
Original line number Diff line number Diff line
@@ -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,

};

include/linux/blkdev.h

+3 −0
Original line number Diff line number Diff line
@@ -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 *);