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

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Message-ID: <20191101192405.GA866154@chrisdown.name>
Date:   Fri, 1 Nov 2019 19:24:05 +0000
From:   Chris Down <chris@...isdown.name>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only

Andrew Morton writes:
>> > 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.
>
>The scenario is that something opens /proc/sys/vm/drop_caches for
>reading, gets unexpected EPERM and blows up?

Right, but...

>OK.  What if we make reads always return "0"?  That will fix the
>misleading output and is more backwards-compatible?

...I'm not convinced that if an application has no error boundary for that 
EPERM that it can tolerate a change in behaviour, either. I mean, if it's 
opening it at all, presumably it intends to do *something* based on the value 
(regardless of import or lack thereof). It may do nothing, but it's not 
possible to know whether that's better or worse than blowing up.

I have mixed feelings on this one. Pragmatically, as someone who programs in 
userspace, I'd like failures based on changes in infrastructure to be loud, not 
silent. If I'm doing something which doesn't work, I'd like to know about it. 
Of course, one can make the argument that as a user of such an application, 
sometimes you don't have that luxury.

Either change is an upgrade from the current situation, at least. I prefer 
towards whatever makes the API the least confusing, which appears to be 
Johannes' original change, but I'd support a patch which always set it to 
0 instead if it was deemed safer.