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From: Andrew Morton <>
To: Chris Down <>
Cc: Johannes Weiner <>,,,
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only
Date: Fri, 1 Nov 2019 12:29:20 -0700
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, 1 Nov 2019 19:24:05 +0000 Chris Down <> 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

  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-31 22:16 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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