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    Subject[PATCH AUTOSEL 5.4 035/266] dm mpath: switch paths in dm_blk_ioctl() code path
    Date
    From: Martin Wilck <mwilck@suse.com>

    [ Upstream commit 2361ae595352dec015d14292f1b539242d8446d6 ]

    SCSI LUN passthrough code such as qemu's "scsi-block" device model
    pass every IO to the host via SG_IO ioctls. Currently, dm-multipath
    calls choose_pgpath() only in the block IO code path, not in the ioctl
    code path (unless current_pgpath is NULL). This has the effect that no
    path switching and thus no load balancing is done for SCSI-passthrough
    IO, unless the active path fails.

    Fix this by using the same logic in multipath_prepare_ioctl() as in
    multipath_clone_and_map().

    Note: The allegedly best path selection algorithm, service-time,
    still wouldn't work perfectly, because the io size of the current
    request is always set to 0. Changing that for the IO passthrough
    case would require the ioctl cmd and arg to be passed to dm's
    prepare_ioctl() method.

    Signed-off-by: Martin Wilck <mwilck@suse.com>
    Reviewed-by: Hannes Reinecke <hare@suse.de>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/md/dm-mpath.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
    index 456d790c918c..f2de4c73cc8f 100644
    --- a/drivers/md/dm-mpath.c
    +++ b/drivers/md/dm-mpath.c
    @@ -1856,7 +1856,7 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
    int r;

    current_pgpath = READ_ONCE(m->current_pgpath);
    - if (!current_pgpath)
    + if (!current_pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
    current_pgpath = choose_pgpath(m, 0);

    if (current_pgpath) {
    --
    2.25.1
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