[Bug Report] Discard bios cannot be correctly merged in blk-mq

[Wang Shanker <shankerwangmiao@gmail.com>]

Hi, all

I'm writing to report my recent findings about the handling of discard 
operations. As indicated by a few tests, discard operation cannot be 
correctly merged, which leads to poor performance of RAID456 on discard
requests. I'm not quite familiar with block subsystem, so please correct
me if there are any mistakes in the following analysis.

In blk_discard_mergable(), we can see the handling of merging discard 
operations goes through different processes, decided by whether we have
more than one queue_max_discard_segments. If the device requires the 
sectors should be contiguous in one discard operation, the merging process
will be the same as that for normal read/write operations. Otherwise, 
bio_attempt_discard_merge will try to merge as many bios as the device
allows, ignoring the contiguity. Sadly, for both cases, there are problems.

For devices requiring contiguous sector ranges(such as scsi disks), 
bio_attempt_front_merge() or bio_attempt_back_merge() will be handling 
the merging process, and both control flows will arrive at 
ll_new_hw_segment(), where the following statement:

    req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req)

can never be true, since blk_rq_get_max_segments(req) will always be 1.
As a result, no discard operations shall be merged.

For devices supporting multiple segments of sector ranges, 
bio_attempt_discard_merge() will take over the process. Indeed it will merge
some bios. But how many bios can be merged into one request? In the 
implementation, the maximum number of bios is limited mainly by 
queue_max_discard_segments (also by blk_rq_get_max_sectors, but it's not where
the problem is). However, it is not the case, since bio_attempt_discard_merge
is not aware of the contiguity of bios. Suppose there are 20 contiguous bios.
They should be considered as only one segment instead 20 of them.

You may wonder the importance of merging discard operations. In the 
implementation of RAID456, bios are committed in 4k trunks (they call
them as stripes in the code and the size is determined by DEFAULT_STRIPE_SIZE).
The proper merging of the bios is of vital importance for a reasonable 
operating performance of RAID456 devices. In fact, I met this problem
when attempting to create a raid5 volume on a bunch of Nvme SSDs enabling trim
support. Someone also reported similar issues in the linux-raid list
(https://www.spinics.net/lists/raid/msg62108.html). In that post, the author
reported that ``lots of small 4k discard requests that get merged into larger
512k chunks submitted to devices". This can be explained by my above discovery
because nvme allows 128 segments at the maximum in a dsm instruction.

The above two scenarios can be reproduced utilizing latest QEMU, with emulated
scsi drives (for the first scenario) or nvme drives (for the second scenario)
and enabling the trace of scsi_disk_emulate_command_UNMAP or pci_nvme_dsm_deallocate.
The detailed process reproducing is as follows:

1. create a rootfs (e.g. using debootstrap) under ./rootfs/ ;
2. obtain a kernel image vmlinuz and generate a initramfs image initrd.img ;
3. create 3 empty sparse disk images:
  # truncate -s 1T disk1 disk2 disk3
4. using the following qemu command to start the guest vm (here 9p is used 
 as the rootfs because we don't want the io operations on the rootfs influence
 the debugging of the block layer of the guest vm)
  # qemu-system-x86_64 \
        -cpu kvm64 -machine pc,accel=kvm -smp cpus=2,cores=2,sockets=1 -m 2G  \
        -chardev stdio,mux=on,id=char0,signal=off  \
        -fsdev local,path=./rootfs,security_model=passthrough,id=rootfs \
        -device virtio-9p,fsdev=rootfs,mount_tag=rootfs \
        -monitor chardev:char0 \
        -device isa-serial,baudbase=1500000,chardev=char0,index=0,id=ttyS0  \
        -nographic \
        -kernel vmlinuz -initrd initrd.img  \
        -append 'root=rootfs rw rootfstype=9p rootflags=trans=virtio,msize=524288 console=ttyS0,1500000 nokaslr' \
        -blockdev driver=raw,node-name=nvme1,file.driver=file,file.filename=disk1 \
        -blockdev driver=raw,node-name=nvme2,file.driver=file,file.filename=disk2 \
        -blockdev driver=raw,node-name=nvme3,file.driver=file,file.filename=disk3 \
        -trace pci_nvme_dsm_deallocate,file=nvmetrace.log \
        -device nvme,drive=nvme1,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME1 \
        -device nvme,drive=nvme2,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME2 \
        -device nvme,drive=nvme3,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME3
5. enable trim support of the raid456 module:
  # modprobe raid456
  # echo Y > /sys/module/raid456/parameters/devices_handle_discard_safely
6. using mdaam to create a raid5 device in the guest vm:
  # mdadm --create --level=5 --raid-devices=3 /dev/md/test /dev/nvme*n1
7. and issue a discard request on the dm device: (limit the size of 
 discard request because discarding all the 2T data is too slow)
  # blkdiscard -o 0 -l 1M -p 1M --verbose /dev/md/test
8. in nvmetrace.log, there are many pci_nvme_dsm_deallocate events of 4k 
 length (nlb 1).

Similarly, the problem with scsi devices can be emulated using the following 
options for qemu:

        -device virtio-scsi,id=scsi \
        -device scsi-hd,drive=nvme1,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME1 \
        -device scsi-hd,drive=nvme2,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME2 \
        -device scsi-hd,drive=nvme3,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME3 \
        -trace scsi_disk_emulate_command_UNMAP,file=scsitrace.log


Despite the discovery, I cannot come up with a proper fix of this issue due
to my lack of familiarity of the block subsystem. I expect your kind feedback
on this. Thanks in advance.

Cheers,

Miao Wang