[PATCH v6 01/11] documentation: Block Device Filtering Mechanism

[Sergei Shtepa <sergei.shtepa@linux.dev>]

From: Sergei Shtepa <sergei.shtepa@veeam.com>

The document contains:
* Describes the purpose of the mechanism
* A little historical background on the capabilities of handling I/O
  units of the Linux kernel
* Brief description of the design
* Reference to interface description

Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
---
 Documentation/block/blkfilter.rst | 66 +++++++++++++++++++++++++++++++
 Documentation/block/index.rst     |  1 +
 MAINTAINERS                       |  6 +++
 3 files changed, 73 insertions(+)
 create mode 100644 Documentation/block/blkfilter.rst

diff --git a/Documentation/block/blkfilter.rst b/Documentation/block/blkfilter.rst
new file mode 100644
index 000000000000..4e148e78f3d4
--- /dev/null
+++ b/Documentation/block/blkfilter.rst
@@ -0,0 +1,66 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+Block Device Filtering Mechanism
+================================
+
+The block device filtering mechanism provides the ability to attach block
+device filters. Block device filters allow performing additional processing
+for I/O units.
+
+Introduction
+============
+
+The idea of handling I/O units on block devices is not new. Back in the
+2.6 kernel, there was an undocumented possibility of handling I/O units
+by substituting the make_request_fn() function, which belonged to the
+request_queue structure. But none of the in-tree kernel modules used this
+feature, and it was eliminated in the 5.10 kernel.
+
+The block device filtering mechanism returns the ability to handle I/O units.
+It is possible to safely attach a filter to a block device "on the fly" without
+changing the structure of the block device's stack.
+
+It supports attaching one filter to one block device, because there is only
+one filter implementation in the kernel yet.
+See Documentation/block/blksnap.rst.
+
+Design
+======
+
+The block device filtering mechanism provides registration and unregistration
+for filter operations. The struct blkfilter_operations contains a pointer to
+the callback functions for the filter. After registering the filter operations,
+the filter can be managed using block device ioctls BLKFILTER_ATTACH,
+BLKFILTER_DETACH and BLKFILTER_CTL.
+
+When the filter is attached, the callback function is called for each I/O unit
+for a block device, providing I/O unit filtering. Depending on the result of
+filtering the I/O unit, it can either be passed for subsequent processing by
+the block layer, or skipped.
+
+The filter can be implemented as a loadable module. In this case, the filter
+module cannot be unloaded while the filter is attached to at least one of the
+block devices.
+
+Interface description
+=====================
+
+The ioctl BLKFILTER_ATTACH allows user-space programs to attach a block device
+filter to a block device. The ioctl BLKFILTER_DETACH allows user-space programs
+to detach it. Both ioctls use &struct blkfilter_name. The ioctl BLKFILTER_CTL
+allows user-space programs to send a filter-specific command. It use &struct
+blkfilter_ctl.
+
+.. kernel-doc:: include/uapi/linux/blk-filter.h
+
+To register in the system, the filter uses the &struct blkfilter_operations,
+which contains callback functions, unique filter name and module owner. When
+attaching a filter to a block device, the filter creates a &struct blkfilter.
+The pointer to the &struct blkfilter allows the filter to determine for which
+block device the callback functions are being called.
+
+.. kernel-doc:: include/linux/blk-filter.h
+
+.. kernel-doc:: block/blk-filter.c
+   :export:
diff --git a/Documentation/block/index.rst b/Documentation/block/index.rst
index 9fea696f9daa..e9712f72cd6d 100644
--- a/Documentation/block/index.rst
+++ b/Documentation/block/index.rst
@@ -10,6 +10,7 @@ Block
    bfq-iosched
    biovecs
    blk-mq
+   blkfilter
    cmdline-partition
    data-integrity
    deadline-iosched
diff --git a/MAINTAINERS b/MAINTAINERS
index 97f51d5ec1cf..c20cbec81b58 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3584,6 +3584,12 @@ M:	Jan-Simon Moeller <jansimon.moeller@gmx.de>
 S:	Maintained
 F:	drivers/leds/leds-blinkm.c
 
+BLOCK DEVICE FILTERING MECHANISM
+M:	Sergei Shtepa <sergei.shtepa@veeam.com>
+L:	linux-block@vger.kernel.org
+S:	Supported
+F:	Documentation/block/blkfilter.rst
+
 BLOCK LAYER
 M:	Jens Axboe <axboe@kernel.dk>
 L:	linux-block@vger.kernel.org
-- 
2.20.1