loop: support different logical block sizes

From: Omar Sandoval <[email protected]>

Hi, everyone,

Here's another attempt at supporting different logical block sizes for loop devices. First, some terminology:

* Logical block size: the lowest possible block size that the storage device can address.
* Physical block size: the lowest possible sector size that the hardware can operate on without reverting to read-modify-write operations. Always greater than or equal to the logical block size.

These are characteristics of the device itself tracked in the request_queue. For loop devices, both are currently always 512 bytes.

struct block_device also has another block size, basically a "soft" block size which is the preferred I/O size and can be changed from userspace. For loop devices, this is PAGE_SIZE if the backing file is a regular file or otherwise the soft block size of the backing block device.

Patch 1 simplifies how the soft block size is set. I'm tempted to not set it at all and use the default of PAGE_SIZE, but maybe someone is depending on inheriting the soft block size from the backing block device...

Patch 2 sets the physical block size of loop devices to a more meaningful value for loop, PAGE_SIZE.

Patch 3 allows us to change the logical block size. It adds a new ioctl instead of the previous approach (which extends LOOP_{GET,SET}_STATUS). Hannes, I assume you had a specific reason for doing it the way you did, care to explain?

Patch 4 is a cleanup.

This is based on my patch reverting the original block size support, targeting 4.14. Thanks!

Omar Sandoval (4):
loop: get rid of lo_blocksize
loop: set physical block size to PAGE_SIZE
loop: add ioctl for changing logical block size
loop: fold loop_switch() into callers

 drivers/block/loop.c      | 112 ++++++++++++++++------------------------------
 drivers/block/loop.h      |   1 -
 include/uapi/linux/loop.h |   1 +
 3 files changed, 40 insertions(+), 74 deletions(-)

2.14.1