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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>
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@ -141,6 +141,15 @@ control of this block device to that new IO scheduler. Note that writing
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an IO scheduler name to this file will attempt to load that IO scheduler
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module, if it isn't already present in the system.
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write_cache (RW)
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----------------
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When read, this file will display whether the device has write back
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caching enabled or not. It will return "write back" for the former
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case, and "write through" for the latter. Writing to this file can
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change the kernels view of the device, but it doesn't alter the
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device state. This means that it might not be safe to toggle the
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setting from "write back" to "write through", since that will also
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eliminate cache flushes issued by the kernel.
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Jens Axboe <jens.axboe@oracle.com>, February 2009
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@ -846,6 +846,32 @@ void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
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}
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EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);
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/**
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* blk_queue_write_cache - configure queue's write cache
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* @q: the request queue for the device
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* @wc: write back cache on or off
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* @fua: device supports FUA writes, if true
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*
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* Tell the block layer about the write cache of @q.
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*/
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void blk_queue_write_cache(struct request_queue *q, bool wc, bool fua)
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{
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spin_lock_irq(q->queue_lock);
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if (wc) {
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queue_flag_set(QUEUE_FLAG_WC, q);
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q->flush_flags = REQ_FLUSH;
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} else
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queue_flag_clear(QUEUE_FLAG_WC, q);
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if (fua) {
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if (wc)
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q->flush_flags |= REQ_FUA;
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queue_flag_set(QUEUE_FLAG_FUA, q);
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} else
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queue_flag_clear(QUEUE_FLAG_FUA, q);
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spin_unlock_irq(q->queue_lock);
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}
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EXPORT_SYMBOL_GPL(blk_queue_write_cache);
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static int __init blk_settings_init(void)
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{
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blk_max_low_pfn = max_low_pfn - 1;
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@ -347,6 +347,38 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
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return ret;
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}
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static ssize_t queue_wc_show(struct request_queue *q, char *page)
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{
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if (test_bit(QUEUE_FLAG_WC, &q->queue_flags))
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return sprintf(page, "write back\n");
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return sprintf(page, "write through\n");
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}
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static ssize_t queue_wc_store(struct request_queue *q, const char *page,
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size_t count)
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{
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int set = -1;
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if (!strncmp(page, "write back", 10))
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set = 1;
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else if (!strncmp(page, "write through", 13) ||
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!strncmp(page, "none", 4))
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set = 0;
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if (set == -1)
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return -EINVAL;
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spin_lock_irq(q->queue_lock);
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if (set)
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queue_flag_set(QUEUE_FLAG_WC, q);
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else
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queue_flag_clear(QUEUE_FLAG_WC, q);
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spin_unlock_irq(q->queue_lock);
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return count;
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}
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static struct queue_sysfs_entry queue_requests_entry = {
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.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
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.show = queue_requests_show,
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@ -478,6 +510,12 @@ static struct queue_sysfs_entry queue_poll_entry = {
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.store = queue_poll_store,
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};
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static struct queue_sysfs_entry queue_wc_entry = {
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.attr = {.name = "write_cache", .mode = S_IRUGO | S_IWUSR },
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.show = queue_wc_show,
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.store = queue_wc_store,
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};
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static struct attribute *default_attrs[] = {
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&queue_requests_entry.attr,
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&queue_ra_entry.attr,
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@ -503,6 +541,7 @@ static struct attribute *default_attrs[] = {
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&queue_iostats_entry.attr,
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&queue_random_entry.attr,
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&queue_poll_entry.attr,
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&queue_wc_entry.attr,
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NULL,
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};
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@ -491,6 +491,8 @@ struct request_queue {
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#define QUEUE_FLAG_INIT_DONE 20 /* queue is initialized */
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#define QUEUE_FLAG_NO_SG_MERGE 21 /* don't attempt to merge SG segments*/
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#define QUEUE_FLAG_POLL 22 /* IO polling enabled if set */
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#define QUEUE_FLAG_WC 23 /* Write back caching */
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#define QUEUE_FLAG_FUA 24 /* device supports FUA writes */
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#define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
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(1 << QUEUE_FLAG_STACKABLE) | \
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@ -1009,6 +1011,7 @@ extern void blk_queue_rq_timed_out(struct request_queue *, rq_timed_out_fn *);
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extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
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extern void blk_queue_flush(struct request_queue *q, unsigned int flush);
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extern void blk_queue_flush_queueable(struct request_queue *q, bool queueable);
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extern void blk_queue_write_cache(struct request_queue *q, bool enabled, bool fua);
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extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
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extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
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