linux-kernel - Re: [RFC PATCH 0/2 v1] Ioctl for reading block queue information

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Date:	Thu, 9 Dec 2010 11:20:50 -0800
From:	Greg KH <gregkh@...e.de>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	tytso@....edu, sandeen@...hat.com, adilger.kernel@...ger.ca,
	hch@...radead.org, axboe@...nel.dk
Subject: Re: [RFC PATCH 0/2 v1] Ioctl for reading block queue information

On Thu, Dec 09, 2010 at 04:25:35PM +0100, Lukas Czerner wrote:
> Hi all,
> 
> For a long time it has been pretty painful to retrieve informations from
> /sys/block/*/queue for particular block device. Not only it is painful
> to retrieve informations within C tool, parsing strings, etc, but one
> have to run into problem even finding the proper path in sysfs.

What's wrong with using libudev?  That should give you all of this
information easily using a .c program without any need to change the
kernel at all.

> This patch set adds new block ioctl BLKGETQUEUEINFO, which is designed to
> retrieve particular block queue attributes. I have tried to not to
> create completely new interface, but since we already have those
> information within block queue sysfs kobjects, rather retrieve it from
> there.

Ick, no, please just use the sysfs files, don't create a new ioctl, they
are horrid.


> 
> So it binds enum blkq_info_type in include/linux/fs.h with
> default_attrs[] definition within block/blk-sysfs.c for ordering
> purposes, so the userspace has reliable way to address specific
> block queue attribute.
> 
> In order to avoid string parsing (given that 99% of block queue sysfs
> attributes are of unsigned long type) I have introduced new sysfs_ops
> member *get which may be used to pass pointer to any type of data (unsigned
> long in this case). Then this get method is used to withdraw information
> through kobject->ktype->sysfs_ops.
> 
> Although there are some downsides of this approach I would like to discuss:
> 
>  1. In current state BLKGETQUEUEINFO and the whole infrastructure allow to
>     retrieve only numeric values (no text), this means we are no able to
>     retrieve schedule information.
> 
>  2. Adding new *get member into sysfs_ops. I see this as the most painful
>     downside, however, when we have more subsystems adding ioctls like this,
>     they can easily use this method, the same way I am using it now.

Nope, don't do this, just use sysfs how it is supposed to be used, from
userspace.

sorry,

greg k-h
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