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

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Date:   Mon, 12 Jun 2023 18:52:43 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Sergei Shtepa <sergei.shtepa@...am.com>, axboe@...nel.dk,
        hch@...radead.org, corbet@....net, snitzer@...nel.org
Cc:     viro@...iv.linux.org.uk, brauner@...nel.org, willy@...radead.org,
        dlemoal@...nel.org, wsa@...nel.org,
        heikki.krogerus@...ux.intel.com, ming.lei@...hat.com,
        gregkh@...uxfoundation.org, linux-block@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v4 01/11] documentation: Block Device Filtering Mechanism

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@...il.com>
> Signed-off-by: Sergei Shtepa <sergei.shtepa@...am.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

                                                  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