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

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Message-ID: <Z7tZTCsQop1Oxk_O@bombadil.infradead.org>
Date: Sun, 23 Feb 2025 09:22:20 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: Ruiwu Chen <rwchen404@...il.com>,
	Joel Granados <joel.granados@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-doc@...r.kernel.org, corbet@....net, viro@...iv.linux.org.uk,
	keescook@...omium.org, zachwade.k@...il.com
Subject: Re: [PATCH v2] drop_caches: re-enable message after disabling

On Sat, Feb 22, 2025 at 04:45:13PM +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

> ---
> v2: - updated Documentation/ to note this new API.
>     - renamed the variable.
>     - rebase this on top of sysctl-next [1].
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/sysctl/sysctl.git/log/?h=sysctl-next
> 
>  Documentation/admin-guide/sysctl/vm.rst | 11 ++++++++++-
>  fs/drop_caches.c                        | 11 +++++++----
>  2 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index f48eaa98d22d..ef73d36e8b84 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -266,7 +266,16 @@ used::
>  	cat (1234): drop_caches: 3
>  
>  These are informational only.  They do not mean that anything is wrong
> -with your system.  To disable them, echo 4 (bit 2) into drop_caches.
> +with your system.
> +
> +To disable informational::
> +
> +	echo 4 > /proc/sys/vm/drop_caches
> +
> +To enable informational::
> +
> +	echo 0 > /proc/sys/vm/drop_caches
> +
>  
>  enable_soft_offline
>  ===================
> diff --git a/fs/drop_caches.c b/fs/drop_caches.c
> index 019a8b4eaaf9..a49af7023886 100644
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -57,7 +57,7 @@ static int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
>  	if (ret)
>  		return ret;
>  	if (write) {
> -		static int stfu;
> +		static bool silent;
>  
>  		if (sysctl_drop_caches & 1) {
>  			lru_add_drain_all();
> @@ -68,12 +68,15 @@ static int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
>  			drop_slab();
>  			count_vm_event(DROP_SLAB);
>  		}
> -		if (!stfu) {
> +		if (!silent) {
>  			pr_info("%s (%d): drop_caches: %d\n",
>  				current->comm, task_pid_nr(current),
>  				sysctl_drop_caches);
>  		}
> -		stfu |= sysctl_drop_caches & 4;
> +		if (sysctl_drop_caches == 0)
> +			silent = true;
> +		else if (sysctl_drop_caches == 4)
> +			silent = false;
>  	}
>  	return 0;
>  }
> @@ -85,7 +88,7 @@ static const struct ctl_table drop_caches_table[] = {
>  		.maxlen		= sizeof(int),
>  		.mode		= 0200,
>  		.proc_handler	= drop_caches_sysctl_handler,
> -		.extra1		= SYSCTL_ONE,
> +		.extra1		= SYSCTL_ZERO,
>  		.extra2		= SYSCTL_FOUR,
>  	},
>  };
> -- 
> 2.27.0
>