This series implements support for ZBC disks used through the scsi-mq I/O path. The current scsi level support of ZBC disks guarantees write request ordering using a per-zone write lock which prevents issuing simultaneously multiple write commands to a zone, doing so avoid reordering of sequential writes to sequential zones. This method is however ineffective when scsi-mq is used with zoned block devices. This is due to the different execution model of blk-mq which passes a request to the scsi layer for dispatching after the request has been removed from the I/O scheduler queue. That is, when the scsi layer tries to lock the target zone of the request, the request may already be out of order and zone write locking fails to prevent that. Various approaches have been tried to solve this problem. All of them had the serious disadvantage of cluttering blk-mq code with zoned block device specific conditions and processing. As such extensive changes can only turn into a maintenance nightmares, a radically different solution is proposed here. This series proposes implementing scsi-mq support for zoned block devices at the I/O scheduler level with simple modifications of the mq-deadline scheduler. the modifications are the addition of a per zone write locking mechanism similar to that implemented in sd_zbc.c for the legacy scsi path. The zone write locking mechanism is used for the exact same purpose, that is, to limit writes per zone to at most one request to avoid reordering. The locking context however changes from that of scsi-sq and is moved to the dispatch_request method of the scheduler. Within this context, under a spin lock guaranteeing atomicity against other dispatch contexts, target zones of write requests can be locked before write requests removal from the scheduler. In effect, this results in the same behavior as the legacy scsi path. Sequential write ordering is preserved. The changes to mq-deadline do not affect regular disks: the same scheduling behavior is maintained for these. The modification are also optimized to not lock conventional zones. To do so, additional data is introduced in the request queue structure so that the low level scsi code can pass upward information such as the total number of zones and zone types of the device. The availability of this new data avoids difficulties in accessing this information from the I/O scheduler initialization method (init_queue() method) context. Of note is that patch 15 of this series removes setting mq-deadline as the default scheduler for block devices with a single hardware queue. The reason for this is that setting the default scheduler is done very early in the device initialization sequence, when the disk characteristics are not yet known. This results in mq-deadline not correctly setting the default zones write locking behavior nased on the device zoning model. Setting of a default I/O scheduler can be done easily with udev rules later in the system initialization process, leading to correct default settings for zoned block devices. Comments are as always very much appreciated. Changes from v3: * Integrated support directly into mq-deadline instead of creating a new I/O scheduler. * Disable setting of default mq scheduler for single queue devices Changes from v2: * Introduced blk_zoned structure * Moved I/O scheduler from drivers/scsi to block Changes from v1: * Addressed Bart's comments for the blk-mq patches (declarations files) * Split (former) patch 4 into multiple patches to facilitate review * Fixed scsi disk lookup from io scheduler by introducing scsi_disk_from_queue() Damien Le Moal (16): scsi: sd_zbc: Move ZBC declarations to scsi_proto.h scsi: sd_zbc: Fix comments and indentation scsi: sd_zbc: Rearrange code scsi: sd_zbc: Use well defined macros scsi: sd_zbc: Fix sd_zbc_read_zoned_characteristics() block: Add zoned block device information to request queue scsi: sd_zbc: Initialize device request queue zoned data scsi: sd_zbc: Limit zone write locking to sequential zones scsi: sd_zbc: Disable zone write locking with scsi-mq block: mq-deadline: Add zoned block device data block: mq-deadline: Introduce zones_wlock attribute blokc: mq-deadline: Introduce dispatch helpers block: mq-deadline: Introduce zone locking support block: mq-deadline: Limit write dispatch for zoned block devices block: do not set mq defaulte scheduler block: mq-deadline: Update documentation Documentation/block/deadline-iosched.txt | 17 ++ block/elevator.c | 17 +- block/mq-deadline.c | 302 +++++++++++++++++++++++++++- drivers/scsi/scsi_lib.c | 5 +- drivers/scsi/sd_zbc.c | 325 ++++++++++++++++++++++++------- include/linux/blkdev.h | 48 +++++ include/scsi/scsi_proto.h | 45 +++-- 7 files changed, 656 insertions(+), 103 deletions(-) -- 2.13.5
