On Fri, 1 Nov 2019 10:45:40 -0400 Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > 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. The scenario is that something opens /proc/sys/vm/drop_caches for reading, gets unexpected EPERM and blows up? > > > 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?"). OK. What if we make reads always return "0"? That will fix the misleading output and is more backwards-compatible?
