[PATCH 0/6] scsi: target: tcmu: Allow data block size greater than PAGE_SIZE

Bodo Stroesser
2021-03-24T19:29:58Z

This series is made on top of Martin's for-next branch.

The data area tcmu uses for data transfer to and from userspace
currently is assigned in data blocks of fixed size PAGE_SIZE.
For big data areas this means, that a big bitmap needs to be
searched for free data blocks. It also means, that the data
buffer for a single scsi cmd can consist of pages which are not
consecutive from userspace point of view.
In that case, userspace receives a long list of IO vecs in the
cmd ring, each vec pointing to another part of the cmd's buffer.

The following patches allow to configure data block size as a
multiple of PAGE_SIZE. For compatibility reasons the default
is 1 * PAGE_SIZE.

Memory allocation for the data area is still done in single
pages. So we do not depend on availability of bigger memory
chunks. Since memory is allocated only once and then re-used
for many commands, it does not cause performance issues.

Advantages of big data blocks:
- bitmap for data block reservation smaller, search of free
blocks faster and less blocks needed (less searches)
- Fewer IO vecs in cmd ring
- If data block size is enhanced to max data tranfer size,
userspace always receives consecutive data buffers with
one IO vec only.

Disadvantage:
- If data block size is big compared to mean size of data
transfers, memory allocation can be higher compared to
data block size = PAGE_SIZE.

Remark:
If this patch set is accepted, I probably have to prepare
a change for rtslib-fb to support new UDEV/config param
data_pages_per_blk (UDEV/info: DataPagesPerBlk)

Bodo Stroesser (6):
scsi: target: tcmu: Adjust names of variables and definitions
scsi: target: tcmu: Prepare for PAGE_SIZE != DATA_BLOCK_SIZE
scsi: target: tcmu: Support DATA_BLOCK_SIZE = N * PAGE_SIZE
scsi: target: tcmu: Remove function tcmu_get_block_page
scsi: target: tcmu: Replace block size definitions with new udev
members
scsi: target: tcmu: Make data_pages_per_blk changeable via configFS

drivers/target/target_core_user.c | 383 +++++++++++++++++++++++---------------
1 file changed, 234 insertions(+), 149 deletions(-)

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2.12.3