iSER on 4.5-rc3 seems to fall over pretty quickly for me.  The
test is a a single LUN, against a Linux iSER target, also running
4.5-rc.  Both are usinx mlx4 hardware.  These messages appear
immediately when running xfstests on xfs (but not during mkfs.xfs),
and after a few dozend tests with ext4, and ultimiately lock up the
system:

Feb 13 13:33:57 redstar37 kernel: [  566.871038] iser: iser_fast_reg_fmr: ib_fmr_pool_map_phys failed: -22
Feb 13 13:33:57 redstar37 kernel: [  566.878278] iser: iser_prepare_read_cmd: Failed to set up Data-IN RDMA
Feb 13 13:33:57 redstar37 kernel: [  566.885604] iser: iser_send_command: conn ffff8809261bbb28 failed task->itt 5 err -22
Feb 13 13:33:57 redstar37 kernel: [  566.899804] iser: iser_fast_reg_fmr: ib_fmr_pool_map_phys failed: -22
Feb 13 13:33:57 redstar37 kernel: [  566.907037] iser: iser_prepare_read_cmd: Failed to set up Data-IN RDMA
Feb 13 13:33:57 redstar37 kernel: [  566.914355] iser: iser_send_command: conn ffff8809261bbb28 failed task->itt 6 err -22

When hacking the initiator to use FRs instead of FMRs I get something
fairly similar:

Feb 13 22:49:18 redstar37 kernel: [  277.500477] iser: iser_fast_reg_mr: failed to map sg (5/6)
Feb 13 22:49:18 redstar37 kernel: [  277.506679] iser: iser_prepare_write_cmd: Failed to register write cmd RDMA mem
Feb 13 22:49:18 redstar37 kernel: [  277.514906] iser: iser_send_command: conn ffff88091ac8cb28 failed task->itt 59 err -22
Feb 13 22:49:18 redstar37 kernel: [  277.529614] iser: iser_fast_reg_mr: failed to map sg (5/6)
Feb 13 22:49:18 redstar37 kernel: [  277.535785] iser: iser_prepare_write_cmd: Failed to register write cmd RDMA mem
Feb 13 22:49:18 redstar37 kernel: [  277.544006] iser: iser_send_command: conn ffff88091ac8cb28 failed task->itt 29 err -22
Feb 13 22:49:18 redstar37 kernel: [  277.558669] iser: iser_fast_reg_mr: failed to map sg (5/6)
Feb 13 22:49:18 redstar37 kernel: [  277.564844] iser: iser_prepare_write_cmd: Failed to register write cmd RDMA mem
Feb 13 22:49:18 redstar37 kernel: [  277.573069] iser: iser_send_command: conn ffff88091ac8cb28 failed task->itt 78 err -22
Feb 13 22:49:18 redstar37 kernel: [  277.587677] iser: iser_fast_reg_mr: failed to map sg (5/6)

The only other kernel version I had available quickly is 3.16 from Debian
Jessie, and that works fine.

Thanks for reporting, I'll have a look.

I suspect this is coming from Keith+Ming changes in
blk_bio_segment_split()...
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