linux-kernel - Re: [PATCH] kernel: sysctl: make drop

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Message-ID: <20191101144540.GA12808@cmpxchg.org>
Date:   Fri, 1 Nov 2019 10:45:40 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only

On Fri, Nov 01, 2019 at 11:09:01AM +0000, Chris Down wrote:
> Hm, not sure why my client didn't show this reply.
> 
> Andrew Morton writes:
> > Risk: some (odd) userspace code will break.  Fixable by manually chmodding
> > it back again.
> 
> The only scenario I can construct in my head is that someone has built
> something to watch drop_caches for modification, but we already have the
> kmsg output for that.
> 
> > Reward: very little.
> > 
> > Is the reward worth the risk?
> 
> There is evidence that this has already caused confusion[0] for many,
> judging by the number of views and votes. I think the reward is higher than
> stated here, since it makes the intent and lack of persistent API in the API
> clearer, and less likely to cause confusion in future.
> 
> 0: https://unix.stackexchange.com/q/17936/10762

Yes, I should have mentioned this in the changelog, but:

While mitigating a VM problem at scale in our fleet, there was
confusion about whether writing to this file will permanently switch
the kernel into a non-caching mode. This influences the decision
making in a tense situation, where tens of people are trying to fix
tens of thousands of affected machines: Do we need a rollback
strategy? What are the performance implications of operating in a
non-caching state for several days? It also caused confusion when the
kernel team said we may need to write the file several times to make
sure it's effective ("But it already reads back 3?").