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    Subject[PATCH 3/6] block: add ability to flag write back caching on a device
    Date
    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.

    Signed-off-by: Jens Axboe <axboe@fb.com>
    ---
    block/blk-settings.c | 11 +++++++++++
    block/blk-sysfs.c | 40 ++++++++++++++++++++++++++++++++++++++++
    include/linux/blkdev.h | 4 ++++
    3 files changed, 55 insertions(+)

    diff --git a/block/blk-settings.c b/block/blk-settings.c
    index c7bb666aafd1..4dbd511a9889 100644
    --- a/block/blk-settings.c
    +++ b/block/blk-settings.c
    @@ -846,6 +846,17 @@ void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
    }
    EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);

    +void blk_queue_write_cache(struct request_queue *q, bool enabled)
    +{
    + spin_lock_irq(q->queue_lock);
    + if (enabled)
    + queue_flag_set(QUEUE_FLAG_WC, q);
    + else
    + queue_flag_clear(QUEUE_FLAG_WC, 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 dd93763057ce..deb2270bf1f3 100644
    --- a/block/blk-sysfs.c
    +++ b/block/blk-sysfs.c
    @@ -347,6 +347,39 @@ 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)
    +{
    + ssize_t ret;
    + 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 ret;
    +}
    +
    static struct queue_sysfs_entry queue_requests_entry = {
    .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
    .show = queue_requests_show,
    @@ -478,6 +511,12 @@ 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 +542,7 @@ 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 930bd4c5b7ff..da5e85c35318 100644
    --- a/include/linux/blkdev.h
    +++ b/include/linux/blkdev.h
    @@ -491,15 +491,18 @@ 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_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
    (1 << QUEUE_FLAG_STACKABLE) | \
    (1 << QUEUE_FLAG_SAME_COMP) | \
    + (1 << QUEUE_FLAG_WC) | \
    (1 << QUEUE_FLAG_ADD_RANDOM))

    #define QUEUE_FLAG_MQ_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
    (1 << QUEUE_FLAG_STACKABLE) | \
    (1 << QUEUE_FLAG_SAME_COMP) | \
    + (1 << QUEUE_FLAG_WC) | \
    (1 << QUEUE_FLAG_POLL))

    static inline void queue_lockdep_assert_held(struct request_queue *q)
    @@ -1009,6 +1012,7 @@ 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);
    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 *);
    --
    2.4.1.168.g1ea28e1
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