From: Andrew Morton <akpm@linux-foundation.org>
To: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only
Date: Fri, 1 Nov 2019 12:29:20 -0700 [thread overview]
Message-ID: <20191101122920.798a6d61b2725da8cfe80549@linux-foundation.org> (raw)
In-Reply-To: <20191101192405.GA866154@chrisdown.name>
On Fri, 1 Nov 2019 19:24:05 +0000 Chris Down <chris@chrisdown.name> wrote:
> 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.
On the other hand.. As I mentioned earlier, if someone's code is
failing because of the permissions change, they can chmod
/proc/sys/vm/drop_caches at boot time and be happy. They have no such
workaround if their software misbehaves due to a read always returning
"0".
next prev parent reply other threads:[~2019-11-01 19:29 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-31 22:16 [PATCH] kernel: sysctl: make drop_caches write-only Johannes Weiner
2019-10-31 23:28 ` Andrew Morton
2019-11-01 11:09 ` Chris Down
2019-11-01 11:09 ` Chris Down
2019-11-01 14:45 ` Johannes Weiner
2019-11-01 18:59 ` Andrew Morton
2019-11-01 19:24 ` Chris Down
2019-11-01 19:29 ` Andrew Morton [this message]
2019-11-01 19:35 ` Andrew Morton
2019-11-02 15:55 ` Alexey Dobriyan
2019-11-03 19:00 ` Eric W. Biederman
2019-11-01 10:58 ` Chris Down
2019-11-04 10:37 ` David Hildenbrand
2019-11-04 13:25 ` Vlastimil Babka
2019-11-05 6:20 ` Michal Hocko
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