linux-kernel - Re: [PATCH 4.19 071/114] kernel: sysctl: make drop

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Message-ID: <20200105154735.GQ16372@sasha-vm>
Date:   Sun, 5 Jan 2020 10:47:35 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Chris Down <chris@...isdown.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4.19 071/114] kernel: sysctl: make drop_caches write-only

On Fri, Jan 03, 2020 at 07:29:11PM +0100, Pavel Machek wrote:
>Hi!
>
>> From: Johannes Weiner <hannes@...xchg.org>
>>
>> [ Upstream commit 204cb79ad42f015312a5bbd7012d09c93d9b46fb ]
>>
>> Currently, the drop_caches proc file and sysctl read back the last value
>> written, suggesting this is somehow a stateful setting instead of a
>> one-time command.  Make it write-only, like e.g.  compact_memory.
>
>I have no problem with that, but is it good idea for stable?

Usually I'd drop it, yes. In this case it seems like it made "real"
users interact with the switch incorrectly, which I parse as a bug.

>Plus, I seem to recall that drop_caches was somehow dangerous,
>debugging-only stuff, one should not use on production system. Did
>that get fixed in the meantime?

Sounds like it's being used in production, at scale, so I'd hope so :)

-- 
Thanks,
Sasha