[PATCH 0/3] dm-verity: optionally use tasklets in dm-verity

[Nathan Huckleberry <nhuck@google.com>]

Using tasklets for disk verification can reduce IO latency.  When there are
accelerated hash instructions it is often better to compute the hash immediately
using a tasklet rather than deferring verification to a work-queue.  This
reduces time spent waiting to schedule work-queue jobs, but requires spending
slightly more time in interrupt context.

A tasklet can only be used for verification if all the required hashes are
already in the dm-bufio cache.  If verification cannot be done in a tasklet, we
fallback the existing work-queue implementation.

To allow tasklets to query the dm-bufio cache, the dm-bufio code must not sleep.
This patchset adds a flag to dm-bufio that disallows sleeping.

The following shows a speed comparison of random reads on a dm-verity device.
The dm-verity device uses a 1G ramdisk for data and a 1G ramdisk for hashes.
One test was run using tasklets and one test was run using the existing
work-queue solution.  Both tests were run when the dm-bufio cache was hot.  The
tasklet implementation performs significantly better since there is no time
waiting for work-queue jobs to be scheduled.

# /data/fio /data/tasklet.fio | grep READ
   READ: bw=181MiB/s (190MB/s), 181MiB/s-181MiB/s (190MB/s-190MB/s), io=512MiB
   (537MB), run=2827-2827msec
# /data/fio /data/workqueue.fio | grep READ
   READ: bw=23.6MiB/s (24.8MB/s), 23.6MiB/s-23.6MiB/s (24.8MB/s-24.8MB/s),
   io=512MiB (537MB), run=21688-21688msec

Nathan Huckleberry (3):
  dm-bufio: Add flags for dm_bufio_client_create
  dm-bufio: Add DM_BUFIO_GET_CANT_SLEEP
  dm-verity: Add try_verify_in_tasklet

 drivers/md/dm-bufio.c                         | 29 +++++--
 drivers/md/dm-ebs-target.c                    |  3 +-
 drivers/md/dm-integrity.c                     |  2 +-
 drivers/md/dm-snap-persistent.c               |  2 +-
 drivers/md/dm-verity-fec.c                    |  4 +-
 drivers/md/dm-verity-target.c                 | 87 ++++++++++++++++---
 drivers/md/dm-verity.h                        |  5 ++
 drivers/md/persistent-data/dm-block-manager.c |  3 +-
 include/linux/dm-bufio.h                      |  8 +-
 9 files changed, 119 insertions(+), 24 deletions(-)

-- 
2.37.1.359.gd136c6c3e2-goog