linux-kernel - Re: kernfs: can read/write method grow buffer size?

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Date:   Fri, 29 Mar 2019 07:22:53 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Marek Behun <marek.behun@....cz>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        Pavel Machek <pavel@....cz>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>
Subject: Re: kernfs: can read/write method grow buffer size?

On Fri, Mar 29, 2019 at 04:09:22AM +0100, Marek Behun wrote:
> Hello Tejun and Greg,
> 
> kernfs_fop_open/read/write allocates a buffer for the ->read, ->write,
> or ->seq_read methods. This buffer is either preallocated or allocated
> on the spot, with minimum size being PAGE_SIZE, if ->atomic_write_len
> is not given.
> 
> There is a question/problem currently in the led-trigger API, that the
> PAGE_SIZE buffer can in some specific scenarios be too short.

And that file is in sysfs?  That's a huge abuse of the sysfs api then :(

> (The trigger file on read returns space separated list of all supported
> triggers, and the currently chosen one is marked specially. The cpu
> activity trigger lists "cpu%i" for all CPU cores, which actually broke
> on some machines with very large number of CPUs. Granted, this could
> have been solved another way (and maybe will be), but we are now
> discussing API for HW LED triggers, which can raise the problem anyway,
> if a specific LED controller supports too many HW LED triggers.)
> 
> Is it allowed to grow this buffer if needed, either via krealloc or by
> creating a special function in kernfs API which does this so that
> led-trigger could use it?
> 
> Or is this completely forbidden?

If this is just for kernfs, and you have your own filesystem, sure, we
can probably do something here.  But if this is for sysfs, no, you all
need to keep to the "one value per file" rule that we have there please.

thanks,

greg k-h