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

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Message-ID: <20190329103252.GA23356@atrey.karlin.mff.cuni.cz>
Date:   Fri, 29 Mar 2019 11:32:52 +0100
From:   Pavel Machek <pavel@....cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Marek Behun <marek.behun@....cz>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org,
        Jacek Anaszewski <jacek.anaszewski@...il.com>
Subject: Re: kernfs: can read/write method grow buffer size?

> On Fri, Mar 29, 2019 at 09:48:23AM +0100, Marek Behun wrote:
> > > pavel@amd:~/cip$ cat /sys/power/state
> > > freeze mem disk
> > > pavel@amd:~/cip$ cat /sys/class/leds/phy0-led/trigger
> > > none bluetooth-power rfkill-any rfkill-none kbd-scrolllock kbd-numlock
> > > kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock
> > > kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock
> > > AC-online BAT0-charging-or-full BAT0-charging BAT0-full
> > > BAT0-charging-blink-full-solid rfkill0 phy0rx phy0tx phy0assoc
> > > phy0radio [phy0tpt] mmc0 timer heartbeat audio-mute audio-micmute
> > > rfkill1 hci0-power rfkill8
> > > pavel@amd:~/cip$
> > > 
> > 
> > Yes, and cpufreq governors too list available governosrs as space
> > separated list.
> > Maybe the "one value per file" rule was thought-of only after these
> > were merged?
> 
> For small numbers of things, like /sys/power/state, which was the first
> file to use this style, it was fine as we "knew" this was going to be a
> small, well-bounded list of states that the file could be in.
> 
> As you have seen, 'trigger' is not that, and I am pretty sure I have
> complained about this in the past.
> 
> I suggest you use a different way of "discovering" what types of
> triggers are available.  I don't know what would work best for you, but
> any time you are ever worried about the size of a sysfs file's buffer,
> you know you are doing something wrong.

Are we doing something wrong?

I don't think so. It looks like sysfs has arbitrary limit it should
not have.

Can we get that fixed? Because userland already knows about this
interface, it is one-type-of-value-per-file, and just removing the
limit seems to be the best way forward.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html