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    Subject[PATCH 2/2] Initialize mempool and elevator only for request-based dm devices
    Date
    Intialize the request_queue and elevator only when the device is marked as
    a request-based device. This avoids unnecessary creation of mempool for
    requests. Also we wrongly initialize the elevator even for bio-based devices.
    As the /sys/block/dm-*/queue/scheduler is exported for device-mapper devices,
    it is possible to confuse with scheduler options for bio-based devices where
    scheduler is not at all used.

    Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

    ---

    diff --git a/drivers/md/dm.c b/drivers/md/dm.c
    index 8a311ea..b01dfbe 100644
    --- a/drivers/md/dm.c
    +++ b/drivers/md/dm.c
    @@ -1749,22 +1749,21 @@ static struct mapped_device *alloc_dev(int minor)
    INIT_LIST_HEAD(&md->uevent_list);
    spin_lock_init(&md->uevent_lock);

    - md->queue = blk_init_queue(dm_request_fn, NULL);
    + md->queue = blk_alloc_queue(GFP_KERNEL);
    if (!md->queue)
    goto bad_queue;

    /*
    * Request-based dm devices cannot be stacked on top of bio-based dm
    - * devices. The type of this dm device has not been decided yet,
    - * although we initialized the queue using blk_init_queue().
    + * devices. The type of this dm device has not been decided yet.
    * The type is decided at the first table loading time.
    * To prevent problematic device stacking, clear the queue flag
    * for request stacking support until then.
    *
    * This queue is new, so no concurrency on the queue_flags.
    */
    + md->queue->queue_flags = QUEUE_FLAG_DEFAULT;
    queue_flag_clear_unlocked(QUEUE_FLAG_STACKABLE, md->queue);
    - md->saved_make_request_fn = md->queue->make_request_fn;
    md->queue->queuedata = md;
    md->queue->backing_dev_info.congested_fn = dm_any_congested;
    md->queue->backing_dev_info.congested_data = md;
    @@ -1772,9 +1771,6 @@ static struct mapped_device *alloc_dev(int minor)
    blk_queue_bounce_limit(md->queue, BLK_BOUNCE_ANY);
    md->queue->unplug_fn = dm_unplug_all;
    blk_queue_merge_bvec(md->queue, dm_merge_bvec);
    - blk_queue_softirq_done(md->queue, dm_softirq_done);
    - blk_queue_prep_rq(md->queue, dm_prep_fn);
    - blk_queue_lld_busy(md->queue, dm_lld_busy);

    md->disk = alloc_disk(1);
    if (!md->disk)
    @@ -2203,7 +2199,25 @@ int dm_swap_table(struct mapped_device *md, struct dm_table *table)
    goto out;
    }

    - __unbind(md);
    + if (md->map)
    + __unbind(md);
    + else {
    + /* new device is being marked as either request-based or bio-based */
    + if (dm_table_request_based(table)) {
    + /* Initialize queue for request-based dm */
    + r = blk_init_allocated_queue(md->queue, dm_request_fn,
    + NULL);
    + if (r)
    + goto out;
    + md->saved_make_request_fn = md->queue->make_request_fn;
    + blk_queue_make_request(md->queue, dm_request);
    + blk_queue_softirq_done(md->queue, dm_softirq_done);
    + blk_queue_prep_rq(md->queue, dm_prep_fn);
    + blk_queue_lld_busy(md->queue, dm_lld_busy);
    +
    + }
    + }
    +
    r = __bind(md, table, &limits);

    out:


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