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Date:   Thu, 31 Oct 2019 16:28:25 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only
Message-Id: <20191031162825.a545a5d4d8567368501769bd@linux-foundation.org>
In-Reply-To: <20191031221602.9375-1-hannes@cmpxchg.org>
References: <20191031221602.9375-1-hannes@cmpxchg.org>
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On Thu, 31 Oct 2019 18:16:02 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> Currently, the drop_caches proc file and sysctl read back the last
> value written, suggesting this is somehow a stateful setting instead
> of a one-time command. Make it write-only, like e.g. compact_memory.
> 
> ...
>
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1474,7 +1474,7 @@ static struct ctl_table vm_table[] = {
>  		.procname	= "drop_caches",
>  		.data		= &sysctl_drop_caches,
>  		.maxlen		= sizeof(int),
> -		.mode		= 0644,
> +		.mode		= 0200,
>  		.proc_handler	= drop_caches_sysctl_handler,
>  		.extra1		= SYSCTL_ONE,
>  		.extra2		= &four,

hm.

Risk: some (odd) userspace code will break.  Fixable by manually chmodding
it back again.

Reward: very little.

Is the reward worth the risk?