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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 0BAC7C3279B for ; Tue, 3 Jul 2018 01:38:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BD406240F8 for ; Tue, 3 Jul 2018 01:38:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BD406240F8 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753667AbeGCBil convert rfc822-to-8bit (ORCPT ); Mon, 2 Jul 2018 21:38:41 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:46336 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753215AbeGCBij (ORCPT ); Mon, 2 Jul 2018 21:38:39 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 37FF0406E81E; Tue, 3 Jul 2018 01:38:39 +0000 (UTC) Received: from llong.remote.csb (ovpn-116-30.sin2.redhat.com [10.67.116.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id D8B792156889; Tue, 3 Jul 2018 01:38:33 +0000 (UTC) Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries To: Andrew Morton , James Bottomley Cc: Linus Torvalds , Al Viro , Linux Kernel Mailing List , linux-fsdevel , Jan Kara , Paul McKenney , Ingo Molnar , Miklos Szeredi , Matthew Wilcox , Larry Woodman , "Wangkai (Kevin,C)" , linux-mm@kvack.org, Michal Hocko References: <1530510723-24814-1-git-send-email-longman@redhat.com> <20180702141811.ef027fd7d8087b7fb2ba0cce@linux-foundation.org> <1530570880.3179.9.camel@HansenPartnership.com> <20180702161925.1c717283dd2bd4a221bc987c@linux-foundation.org> From: Waiman Long Organization: Red Hat Message-ID: <6e86c673-af14-2111-ea30-dc6cac655414@redhat.com> Date: Tue, 3 Jul 2018 09:38:31 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <20180702161925.1c717283dd2bd4a221bc987c@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Tue, 03 Jul 2018 01:38:39 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Tue, 03 Jul 2018 01:38:39 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'longman@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/03/2018 07:19 AM, Andrew Morton wrote: > On Mon, 02 Jul 2018 15:34:40 -0700 James Bottomley wrote: > >> On Mon, 2018-07-02 at 14:18 -0700, Andrew Morton wrote: >>> On Mon, 2 Jul 2018 12:34:00 -0700 Linus Torvalds >> dation.org> wrote: >>> >>>> On Sun, Jul 1, 2018 at 10:52 PM Waiman Long >>>> wrote: >>>>> A rogue application can potentially create a large number of >>>>> negative >>>>> dentries in the system consuming most of the memory available if >>>>> it >>>>> is not under the direct control of a memory controller that >>>>> enforce >>>>> kernel memory limit. >>>> I certainly don't mind the patch series, but I would like it to be >>>> accompanied with some actual example numbers, just to make it all a >>>> bit more concrete. >>>> >>>> Maybe even performance numbers showing "look, I've filled the >>>> dentry >>>> lists with nasty negative dentries, now it's all slower because we >>>> walk those less interesting entries". >>>> >>> (Please cc linux-mm@kvack.org on this work) >>> >>> Yup. The description of the user-visible impact of current behavior >>> is far too vague. >>> >>> In the [5/6] changelog it is mentioned that a large number of -ve >>> dentries can lead to oom-killings. This sounds bad - -ve dentries >>> should be trivially reclaimable and we shouldn't be oom-killing in >>> such a situation. >> If you're old enough, it's déjà vu; Andrea went on a negative dentry >> rampage about 15 years ago: >> >> https://lkml.org/lkml/2002/5/24/71 > That's kinda funny. > >> I think the summary of the thread is that it's not worth it because >> dentries are a clean cache, so they're immediately shrinkable. > Yes, "should be". I could understand that the presence of huge > nunmbers of -ve dentries could result in undesirable reclaim of > pagecache, etc. Triggering oom-killings is very bad, and presumably > has the same cause. > > Before we go and add a large amount of code to do the shrinker's job > for it, we should get a full understanding of what's going wrong. Is > it because the dentry_lru had a mixture of +ve and -ve dentries? > Should we have a separate LRU for -ve dentries? Are we appropriately > aging the various dentries? etc. I have actually investigated having a separate LRU for negative dentries. That will result in a far more invasive patch that will be more disruptive. Another change that was suggested by a colleague is to put a newly created -ve dentry to the tail (LRU end) of the LRU and move it to the head only if it is accessed a second time. That will put most of the negative dentries at the tail that will be more easily trimmed away. Cheers, Longman