From: Johannes Weiner <email@example.com> To: Chris Down <firstname.lastname@example.org> Cc: Andrew Morton <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only Date: Fri, 1 Nov 2019 10:45:40 -0400 Message-ID: <20191101144540.GA12808@cmpxchg.org> (raw) In-Reply-To: <20191101110901.GB690103@chrisdown.name> 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 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?").
next prev parent 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 [this message] 2019-11-01 18:59 ` Andrew Morton 2019-11-01 19:24 ` Chris Down 2019-11-01 19:29 ` Andrew Morton 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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