[PATCH v5 01/11] documentation: Block Device Filtering Mechanism

Sergei Shtepa
2023-06-12T13:53:12Z

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

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
---
Documentation/block/blkfilter.rst | 64 +++++++++++++++++++++++++++++++
Documentation/block/index.rst | 1 +
MAINTAINERS | 6 +++
3 files changed, 71 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..555625789244
--- /dev/null
+++ b/Documentation/block/blkfilter.rst
@@ -0,0 +1,64 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+Block Device Filtering Mechanism
+================================
+
+The block device filtering mechanism is an API that allows to attach block
+device filters. Block device filters allow perform 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 filter to a block device "on the fly" without
+changing the structure of block devices 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,
+filter can be managed using block device ioctl 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 and BLKFILTER_DETACH use structure blkfilter_name.
+It allows to attach a filter to a block device or detach it.
+
+The ioctl BLKFILTER_CTL use structure blkfilter_ctl. It allows to send a
+filter-specific command.
+
+.. kernel-doc:: include/uapi/linux/blk-filter.h
+
+To register in the system, the filter creates its own account, which contains
+callback functions, unique filter name and module owner. This filter account is
+used by the registration functions.
+
+.. 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 e0ad886d3163..d801b8985b43 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3580,6 +3580,12 @@ 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