The following commit was added in bug fa55b5d226dd.

7ac257b862f2c (“blk-mq: remove the request_list usage”)

This commit cleans up unused code in block-mq (mq uses pre-allocated
tags to allocate request, not request_list as it's used only in legacy

A test kernel with a revert of this commit would prove if it is the
cause of this regression.

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   [linux-azure] Storage performance drop on RAID

Status in linux-azure package in Ubuntu:

Bug description:
  In Azure, FIO 4k tests are showing performance drops on latest proposed 
4.15.0 linux-azure kernels - from 52K IOPS to ~38K IOPS (max value reached on 
both sequential and random read tests).
  The setup used is 12 disks in RAID0.
  The affected kernels are:
  Ubuntu 14.04 + 4.15.0-1043
  Ubuntu 16.04 + 4.15.0-1044

  Previous kernel versions had reached max IOPS, ~52K IOPS on both read
  tests (e.g. 16.04 + 4.15.0-1043 kernel). Right now, it seems like the
  issue is in the diff from 1043 kernel on trusty vs 1044 kernel on

  The 52K IOPS is expected at qdepth=256. The repro cmd (as ran by the 
automation) is this:
  fio --size=1023G --direct=1 --ioengine=libaio --filename=/dev/md0 
--overwrite=1 --readwrite=randread --bs=4K --runtime=300 --iodepth=32 
--numjob=8 --output-format=json 
--output=/root/FIOLog/jsonLog/fio-result-randread-4K-256td.json --name='repro'

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