Re: [PATCH v4 01/11] documentation: Block Device Filtering Mechanism

From: Sergei Shtepa
Date: Tue Jun 13 2023 - 11:24:23 EST

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In reply to: Randy Dunlap: "Re: [PATCH v4 01/11] documentation: Block Device Filtering Mechanism"
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Thank you very much, Randy, for the work you've done.
Sometimes I want to compile the documentation into bytecode, fix the warnings
and debug it in the debugger.

On 6/13/23 03:52, Randy Dunlap wrote:
> Subject:
> Re: [PATCH v4 01/11] documentation: Block Device Filtering Mechanism
> From:
> Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Date:
> 6/13/23, 03:52
>
> To:
> Sergei Shtepa <sergei.shtepa@xxxxxxxxx>, axboe@xxxxxxxxx, hch@xxxxxxxxxxxxx, corbet@xxxxxxx, snitzer@xxxxxxxxxx
> CC:
> viro@xxxxxxxxxxxxxxxxxx, brauner@xxxxxxxxxx, willy@xxxxxxxxxxxxx, dlemoal@xxxxxxxxxx, wsa@xxxxxxxxxx, heikki.krogerus@xxxxxxxxxxxxxxx, ming.lei@xxxxxxxxxx, gregkh@xxxxxxxxxxxxxxxxxxx, linux-block@xxxxxxxxxxxxxxx, linux-doc@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, Bagas Sanjaya <bagasdotme@xxxxxxxxx>
>
>
> Hi--
>
> On 6/9/23 04:58, Sergei Shtepa wrote:
>> 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@xxxxxxxxx>
>> Signed-off-by: Sergei Shtepa <sergei.shtepa@xxxxxxxxx>
>> ---
>> 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
> that allows {what or who} to attach
>
> so who/what does the attach? Is it a driver or a user or admin or something else?
> and attach them to what?
>
>> +device filters. Block device filters allow perform additional processing
> allow performing ...
>
>> +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
> attach a filter
> or
> attach filters
>
>> +changing the structure of block devices stack.
> 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,
>> +filter can be managed using block device ioctl BLKFILTER_ATTACH,
> a filter
> or
> the filter ioctls
>
>> +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.
> ioctls
>
>> +It allows to attach a filter to a block device or detach it.
> It allows a driver to attach a filter ...
> ?
>
>> +
>> +The ioctl BLKFILTER_CTL use structure blkfilter_ctl. It allows to send a
> It allows a driver 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.
> I'm having a problem with this "account" thingy. Can you explain more about it?
> Is there an alternate word that might be used here?
>
>> +
>> +.. kernel-doc:: include/linux/blk-filter.h
>> +
>> +.. kernel-doc:: block/blk-filter.c
>> + :export:
> Thanks.
> -- ~Randy
>

fix of the blkfilter.rst document and help in Kconfig for blksnap

Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Signed-off-by: Sergei Shtepa <sergei.shtepa@xxxxxxxxx>
---
Documentation/block/blkfilter.rst | 28 +++++++++++++++-------------
drivers/block/blksnap/Kconfig | 2 +-
2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/Documentation/block/blkfilter.rst b/Documentation/block/blkfilter.rst
index 555625789244..b5160d1008fd 100644
--- a/Documentation/block/blkfilter.rst
+++ b/Documentation/block/blkfilter.rst
@@ -4,8 +4,8 @@
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
+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
@@ -18,8 +18,8 @@ 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 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.
@@ -31,7 +31,7 @@ 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,
+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
@@ -46,17 +46,19 @@ 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.
+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 structure blkfilter_name. The ioctl BLKFILTER_CTL
+allows user-space programs to send a filter-specific command. It use structure
+blkfilter_ctl.

.. 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.
+To register in the system, the filter uses the blkfilter_operations structure,
+which contains callback functions, unique filter name and module owner. When
+attaching a filter to a block device, the filter creates a blkfilter structure.
+The pointer to the blkfilter structure allows the filter to determine for
+which block device the callback functions are being called.

.. kernel-doc:: include/linux/blk-filter.h

diff --git a/drivers/block/blksnap/Kconfig b/drivers/block/blksnap/Kconfig
index 14081359847b..11df0886489d 100644
--- a/drivers/block/blksnap/Kconfig
+++ b/drivers/block/blksnap/Kconfig
@@ -8,5 +8,5 @@ config BLKSNAP
help
Allow to create snapshots and track block changes for block devices.
Designed for creating backups for simple block devices. Snapshots are
- temporary and are released then backup is completed. Change block
+ temporary and are released when backup is completed. Change block
tracking allows to create incremental or differential backups.
--
2.20.1

Next message: Rafael J. Wysocki: "[PATCH v1] ACPI: sleep: Avoid breaking S3 wakeup due to might_sleep()"
Previous message: Arnaldo Carvalho de Melo: "Re: [PATCH 5/5] perf arm-spe: Fix a dangling Documentation/arm64 reference"
In reply to: Randy Dunlap: "Re: [PATCH v4 01/11] documentation: Block Device Filtering Mechanism"
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