From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 86F5B6B501D for ; Thu, 30 Aug 2018 03:20:59 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id z30-v6so3255740edd.19 for ; Thu, 30 Aug 2018 00:20:59 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id k5-v6si4963788edd.380.2018.08.30.00.20.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Aug 2018 00:20:58 -0700 (PDT) Date: Thu, 30 Aug 2018 09:20:56 +0200 From: Michal Hocko Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed Message-ID: <20180830072056.GC2656@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <374c2c5c-cc9b-af03-a800-32f2cf8a3055@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Waiman Long 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)" 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. -- Michal Hocko SUSE Labs