From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AD56CCA9ECE for ; Thu, 31 Oct 2019 23:28:28 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 724DD20873 for ; Thu, 31 Oct 2019 23:28:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="Y7jGEoeq" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 724DD20873 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 058C46B0005; Thu, 31 Oct 2019 19:28:28 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 009A06B0006; Thu, 31 Oct 2019 19:28:27 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E612A6B0007; Thu, 31 Oct 2019 19:28:27 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0156.hostedemail.com [216.40.44.156]) by kanga.kvack.org (Postfix) with ESMTP id C3E8C6B0005 for ; Thu, 31 Oct 2019 19:28:27 -0400 (EDT) Received: from smtpin05.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id 6FFC518024A4F for ; Thu, 31 Oct 2019 23:28:27 +0000 (UTC) X-FDA: 76105671054.05.back21_49cd71dc11c5b X-HE-Tag: back21_49cd71dc11c5b X-Filterd-Recvd-Size: 2313 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf36.hostedemail.com (Postfix) with ESMTP for ; Thu, 31 Oct 2019 23:28:27 +0000 (UTC) Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E34AC2086D; Thu, 31 Oct 2019 23:28:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572564506; bh=PBIVho+NsCRnh1duj1FgKfPosLuMmKWV+Ax7lihqGaM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Y7jGEoeqAAEdG8wkoJsNM09wjH7WM7Ye0HkBYw1Zgx9Fs8zFjBRuIvcG2VtctppWL dOCPw42Jl+NFLlSxc9Irhco9vXW/+z7XAo+8yPmdzxKaMf+PXZbFrJx0A90OVcfnlq gDs9SMtGGmjxRfb6THajuRWXupQTlLp4Zux2BfeI= Date: Thu, 31 Oct 2019 16:28:25 -0700 From: Andrew Morton To: Johannes Weiner Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only Message-Id: <20191031162825.a545a5d4d8567368501769bd@linux-foundation.org> In-Reply-To: <20191031221602.9375-1-hannes@cmpxchg.org> References: <20191031221602.9375-1-hannes@cmpxchg.org> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 31 Oct 2019 18:16:02 -0400 Johannes Weiner wrote: > 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. > > ... > > --- a/kernel/sysctl.c > +++ b/kernel/sysctl.c > @@ -1474,7 +1474,7 @@ static struct ctl_table vm_table[] = { > .procname = "drop_caches", > .data = &sysctl_drop_caches, > .maxlen = sizeof(int), > - .mode = 0644, > + .mode = 0200, > .proc_handler = drop_caches_sysctl_handler, > .extra1 = SYSCTL_ONE, > .extra2 = &four, hm. Risk: some (odd) userspace code will break. Fixable by manually chmodding it back again. Reward: very little. Is the reward worth the risk?