On Wed, Apr 13, 2016 at 2:35 AM, Jens Axboe <axboe@xxxxxx> wrote:
> 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@xxxxxx>
Looks a very nice cleanup! But I have just one question.
> ---
> Documentation/block/queue-sysfs.txt | 9 +++++++++
> block/blk-settings.c | 26 +++++++++++++++++++++++++
> block/blk-sysfs.c | 39 +++++++++++++++++++++++++++++++++++++
> include/linux/blkdev.h | 3 +++
> 4 files changed, 77 insertions(+)
>
> diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
> index e5d914845be6..dce25d848d92 100644
> --- 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@xxxxxxxxxx>, February 2009
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 331e4eee0dda..c903bee43cf8 100644
> --- 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 995b58d46ed1..99205965f559 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);
When the write cache (queue)flag is changed, I guess we need to change
q->flush_flags via blk_queue_write_cache() too? Otherwise, the change
on write cache flag can't affect block flush behaviour.
BTW, looks the flush_flags has been redundant after this patch, can
we just kill it in the future?
Thanks,
> +
> + 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 669e419d6234..18b7ff2184dd 100644
> --- 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 *);
> --
> 2.8.0.rc4.6.g7e4ba36
>
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--
Ming Lei
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