linux-kernel - Re: [PATCH v2] drop_caches: re-enable message after disabling

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Message-ID: <Z9IQ2jam9hkXnPhg@bombadil.infradead.org>
Date: Wed, 12 Mar 2025 15:55:22 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: Joel Granados <joel.granados@...nel.org>
Cc: Ruiwu Chen <rwchen404@...il.com>, corbet@....net, keescook@...omium.org,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	zachwade.k@...il.com
Subject: Re: [PATCH v2] drop_caches: re-enable message after disabling

On Mon, Mar 10, 2025 at 02:51:11PM +0100, Joel Granados wrote:
> On Sat, Mar 08, 2025 at 04:05:49PM +0800, Ruiwu Chen wrote:
> > >> When 'echo 4 > /proc/sys/vm/drop_caches' the message is disabled,
> > >> but there is no interface to enable the message, only by restarting
> > >> the way, so add the 'echo 0 > /proc/sys/vm/drop_caches' way to
> > >> enabled the message again.
> > >> 
> > >> Signed-off-by: Ruiwu Chen <rwchen404@...il.com>
> > >
> > > You are overcomplicating things, if you just want to re-enable messages
> > > you can just use:
> > >
> > > -		stfu |= sysctl_drop_caches & 4;
> > > +		stfu = sysctl_drop_caches & 4;
> > >
> > > The bool is there as 4 is intended as a bit flag, you can can figure
> > > out what values you want and just append 4 to it to get the expected
> > > result.
> > >
> > >  Luis
> > 
> > Is that what you mean ?
> > 
> > -               stfu |= sysctl_drop_caches & 4;
> > +               stfu ^= sysctl_drop_caches & 4;
> > 
> > 'echo 4 > /sys/kernel/vm/drop_caches' can disable or open messages,
> > This is what I originally thought, but there is uncertainty that when different operators execute the command,
> > It is not possible to determine whether this time is enabled or turned on unless you operate it twice.
> 
> So can you use ^= or not?

No,  ^= does not work, see a boolean truth table.

> And what does operate it twice mean?

I think the reporter meant an "sysadmin", say two folks admining a system.
Since we this as a flag to enable disabling it easily we can just
always check for the flag as I suggested:

stfu = sysctl_drop_caches & 4

  Luis