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 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 180C0C433F4 for ; Thu, 30 Aug 2018 21:48:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C0C8F20658 for ; Thu, 30 Aug 2018 21:48:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C0C8F20658 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 S1727794AbeHaBxG (ORCPT ); Thu, 30 Aug 2018 21:53:06 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:34816 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727185AbeHaBxG (ORCPT ); Thu, 30 Aug 2018 21:53:06 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0714B8163C40; Thu, 30 Aug 2018 21:48:52 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id C140B2027056; Thu, 30 Aug 2018 21:48:50 +0000 (UTC) Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed To: Michal Hocko Cc: Alexander Viro , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, "Luis R. Rodriguez" , Kees Cook , Linus Torvalds , Jan Kara , "Paul E. McKenney" , Andrew Morton , Ingo Molnar , Miklos Szeredi , Matthew Wilcox , Larry Woodman , James Bottomley , "Wangkai (Kevin C)" References: <1535476780-5773-1-git-send-email-longman@redhat.com> <1535476780-5773-3-git-send-email-longman@redhat.com> <20180829075129.GU10223@dhcp22.suse.cz> <374c2c5c-cc9b-af03-a800-32f2cf8a3055@redhat.com> <20180830072056.GC2656@dhcp22.suse.cz> From: Waiman Long Organization: Red Hat Message-ID: <9b5c3e96-9dcc-e601-9d15-116aef4bdbfb@redhat.com> Date: Thu, 30 Aug 2018 17:48:50 -0400 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: <20180830072056.GC2656@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Thu, 30 Aug 2018 21:48:52 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Thu, 30 Aug 2018 21:48:52 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.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 08/30/2018 03:20 AM, Michal Hocko wrote: > On Wed 29-08-18 15:58:52, Waiman Long wrote: >> On 08/29/2018 03:51 AM, Michal Hocko wrote: >>> On Tue 28-08-18 13:19:40, Waiman Long wrote: >>>> For negative dentries that are accessed once and never used again, they >>>> should be removed first before other dentries when shrinker is running. >>>> This is done by putting negative dentries at the head of the LRU list >>>> instead at the tail. >>>> >>>> A new DCACHE_NEW_NEGATIVE flag is now added to a negative dentry when it >>>> is initially created. When such a dentry is added to the LRU, it will be >>>> added to the head so that it will be the first to go when a shrinker is >>>> running if it is never accessed again (DCACHE_REFERENCED bit not set). >>>> The flag is cleared after the LRU list addition. >>> Placing object to the head of the LRU list can be really tricky as Dave >>> pointed out. I am not familiar with the dentry cache reclaim so my >>> comparison below might not apply. Let me try anyway. >>> >>> Negative dentries sound very similar to MADV_FREE pages from the reclaim >>> POV. They are primary candidate for reclaim, yet you want to preserve >>> aging to other easily reclaimable objects (including other MADV_FREE >>> pages). What we do for those pages is to move them from the anonymous >>> LRU list to the inactive file LRU list. Now you obviously do not have >>> anon/file LRUs but something similar to active/inactive LRU lists might >>> be a reasonably good match. Have easily reclaimable dentries on the >>> inactive list including negative dentries. If negative entries are >>> heavily used then they can promote to the active list because there is >>> no reason to reclaim them soon. >>> >>> Just my 2c >> As mentioned in my reply to Dave, I did considered using a 2 LRU list >> solution. However, that will add more complexity to the dcache LRU >> management code than my current approach and probably more potential for >> slowdown. > I completely agree with Dave here. This is not easy but trying to sneak > in something that works for an _artificial_ workload is simply a no go. > So if it takes to come with a more complex solution to cover more > general workloads then be it. Someone has to bite a bullet and explore > that direction. It won't be a simple project but well, if negative > dentries really matter then it is worth making the reclaim design robust > and comprehensible rather than adhoc and unpredictable. OK, I will need to spend more time to think about a better way of doing that. Cheers, Longman