[PATCH v4 0/6] block-layer interposer

[Sergei Shtepa <sergei.shtepa@veeam.com>]

Hi all.

I'm ready to suggest the block-layer interposer (blk_interposer).
blk_interposer allows to intercept bio requests, remap bio to another
devices or add new bios.

This patch series adds support blk_interposer for dm-linear.

In the first patch, I suggest the remap_and_filter.rst file.
Yes, Mike, it's probably too early for documentation, but maybe it will be
interesting for someone. In the documentation I tried to explain
the purpose of blk_interposer and what prospects it opens up.

The second patch is dedicated to blk_interposer itself, which provides
the ability to intercept bio.

The third - adds the function blk_mq_is_queue_frozen(). It allows to
assert a queue state.

The fourth one adds support for blk_interposer from the device mapper.
Added ioctl DM_DEV_REMAP_CMD.

In the fifth - added the 'noexcl' option for dm-linear, which allows
to open the underlying block-device without the FMODE_EXCL mode.
This allows to create a dm device to which can redirect bio requests
using DM_DEV_REMAP_CMD.

Here I would like to discuss the implementation.
I don't like to play with flags. Adding the non_exclusive flags creates
an excessive complexity. An alternative solution is to explicitly specify
the FMODE_EXCL mode for each dm-target. The problem is that in this case,
we will have to change each dm-target. It doesn't look cool either.

The latest patch changes linear.rst with the description of the 'noexcl'
option that is added for dm-linear.

A little history of changes:

v4 - current patch set
Mostly changes were made, due to Damien's comments:
 * on the design of the code;
 * by the patch set organization;
 * bug with passing a wrong parameter to dm_get_device();
 * description of the 'noexcl' parameter in the linear.rst.
Also added remap_and_filter.rst.

v3 - https://patchwork.kernel.org/project/linux-block/cover/1611853955-32167-1-git-send-email-sergei.shtepa@veeam.com/
In this version, I already suggested blk_interposer to apply to dm-linear.
Problems were solved:
 * Interception of bio requests from a specific device on the disk, not
   from the entire disk. To do this, we added the dm_interposed_dev
   structure and an interval tree to store these structures.
 * Implemented ioctl DM_DEV_REMAP_CMD. A patch with changes in the lvm2
   project was sent to the team lvm-devel@redhat.com.
 * Added the 'noexcl' option for dm-linear, which allows you to open
   the underlying block-device without FMODE_EXCL mode.

v2 - https://patchwork.kernel.org/project/linux-block/cover/1607518911-30692-1-git-send-email-sergei.shtepa@veeam.com/
I tried to suggest blk_interposer without using it in device mapper,
but with the addition of a sample of its use. It was then that I learned
about the maintainers' attitudes towards the samples directory :).

v1 - https://lwn.net/ml/linux-block/20201119164924.74401-1-hare@suse.de/
This Hannes's patch can be considered as a starting point, since this is
where the interception mechanism and the term blk_interposer itself
appeared. It became clear that blk_interposer can be useful for
device mapper.

before v1 - https://patchwork.kernel.org/project/linux-block/cover/1603271049-20681-1-git-send-email-sergei.shtepa@veeam.com/
I tried to offer a rather cumbersome blk-filter and a monster-like
blk-snap module for creating snapshots.

Thank you to everyone who was able to take the time to review
the previous versions.
I hope that this time I achieved the required quality.

Thanks,
Sergei.

Sergei Shtepa (6):
  docs: device-mapper: add remap_and_filter
  block: add blk_interposer
  block: add blk_mq_is_queue_frozen()
  dm: new ioctl DM_DEV_REMAP_CMD
  dm: add 'noexcl' option for dm-linear
  docs: device-mapper: 'noexcl' option for dm-linear

 .../admin-guide/device-mapper/index.rst       |   1 +
 .../admin-guide/device-mapper/linear.rst      |  26 +-
 .../device-mapper/remap_and_filter.rst        | 132 ++++++
 block/bio.c                                   |   2 +
 block/blk-core.c                              |  33 ++
 block/blk-mq.c                                |  13 +
 block/genhd.c                                 |  82 ++++
 drivers/md/dm-core.h                          |  20 +
 drivers/md/dm-ioctl.c                         |  39 ++
 drivers/md/dm-linear.c                        |  14 +-
 drivers/md/dm-table.c                         |  12 +-
 drivers/md/dm.c                               | 400 +++++++++++++++++-
 drivers/md/dm.h                               |   2 +-
 include/linux/blk-mq.h                        |   1 +
 include/linux/blk_types.h                     |   6 +-
 include/linux/device-mapper.h                 |   7 +
 include/linux/genhd.h                         |  18 +
 include/uapi/linux/dm-ioctl.h                 |  15 +-
 18 files changed, 792 insertions(+), 31 deletions(-)
 create mode 100644 Documentation/admin-guide/device-mapper/remap_and_filter.rst

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2.20.1