tree 5eda8e4bc4e7248607e584428a1044dd31cd4130
parent 68ecd85e279f7b381e7f6ae6ee18a1f53dba1bb1
author Damien Le Moal <damien.lemoal@wdc.com> 1550722265 +0900
committer Jens Axboe <axboe@kernel.dk> 1550981941 -0700

zbd: Fix zone locking for async I/O engines

For a zoned block device with zonemode=zbd, the lock on the target zone
of an I/O is held from the time the I/O is prepared with
zbd_adjust_block() execution in fill_io_u() until the I/O is queued in
td_io_queue(). For a sync I/O engines, this means that the target zone
of an I/O operations is locked throughout the liftime of an I/O unit,
resulting in the serialization of write request preparation and
execution, as well as serialization of write operations and reset zone
operations for a zone, avoiding error inducing reordering.

However, in the case of an async I/O engine, the engine ->commit()
method falls outside of the zone lock serialization for all I/O units
that will be issued by the method execution. This results in potential
reordering of write requests during issuing, as well as simultaneous
queueing of write requests and zone reset operations resulting in
unaligned write errors.

For example, using a 1GB null_blk zoned device, the command:

fio --name=nullb0 --filename=/dev/nullb0 --direct=1 --zonemode=zbd
    --bs=4k --rw=randwrite --ioengine=libaio --group_reporting=1
    --iodepth=32 --numjobs=4

always fails due to unaligned write errors.

Fix this by refining the control over zone locking and unlocking.
Locking of an I/O target zone is unchanged and done in
zbd_adjust_block(), but the I/O callback function zbd_post_submit()
which updates a zone write pointer and unlocks the zone is split into
two different callbacks zbd_queue_io() and zbd_put_io().
zbd_queue_io() updates the zone write pointer for write operations and
unlocks the target zone only if the I/O operation was not queued or if
the I/O operation completed during the execution of the engine
->queue() method (e.g. a sync I/O engine is being used). The execution
of this I/O callback is done right after executing the I/O engine
->queue() method. The zbd_put_io() callback is used to unlock an I/O
target zone after completion of an I/O from within the put_io_u()
function.

To simplify the code the helper functions zbd_queue_io_u() and
zbd_put_io_u() which respectively call an io_u zbd_queue_io() and
zbd_put_io() callbacks are introduced. These helper functions are
conditionally defined only if CONFIG_LINUX_BLKZONED is set.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>