From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f198.google.com (mail-qt0-f198.google.com [209.85.216.198]) by kanga.kvack.org (Postfix) with ESMTP id 44AEE6B0266 for ; Wed, 18 Jul 2018 12:01:09 -0400 (EDT) Received: by mail-qt0-f198.google.com with SMTP id l13-v6so3526713qth.8 for ; Wed, 18 Jul 2018 09:01:09 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id a18-v6si1391830qtm.396.2018.07.18.09.01.05 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 18 Jul 2018 09:01:05 -0700 (PDT) Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries 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> <20180703091821.oiywpdxd6rhtxl4p@quack2.suse.cz> <20180714173516.uumlhs4wgfgrlc32@devuan> <20180714183445.GJ30522@ZenIV.linux.org.uk> From: Waiman Long Message-ID: <990ac8fd-69a6-7d6b-6608-cda012ac22a4@redhat.com> Date: Wed, 18 Jul 2018 12:01:03 -0400 MIME-Version: 1.0 In-Reply-To: <20180714183445.GJ30522@ZenIV.linux.org.uk> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Al Viro , Linus Torvalds Cc: Pavel Machek , Jan Kara , Andrew Morton , James Bottomley , Linux Kernel Mailing List , linux-fsdevel , Paul McKenney , Ingo Molnar , Miklos Szeredi , Matthew Wilcox , Larry Woodman , "Wangkai (Kevin,C)" , linux-mm , Michal Hocko On 07/14/2018 02:34 PM, Al Viro wrote: > On Sat, Jul 14, 2018 at 11:00:32AM -0700, Linus Torvalds wrote: >> On Sat, Jul 14, 2018 at 10:35 AM Pavel Machek wrote: >>> Could we allocate -ve entries from separate slab? >> No, because negative dentrires don't stay negative. >> >> Every single positive dentry starts out as a negative dentry that is >> passed in to "lookup()" to maybe be made positive. >> >> And most of the time they do turn positive, because most of the= >> time people actually open files that exist. >> >> But then occasionally you don't, because you're just blindly opening a= >> filename whether it exists or not (to _check_ whether it's there). > BTW, one point that might not be realized by everyone: negative dentrie= s > are *not* the hard case. > mount -t tmpfs none /mnt > touch /mnt/a > for i in `seq 100000`; do ln /mnt/a /mnt/$i; done > > and you've got 100000 *unevictable* dentries, with the time per iterati= on > being not all that high (especially if you just call link(2) in a loop)= =2E > They are all positive and all pinned. And you've got only one inode > there and no persistently opened files, so rlimit and quota won't help > any. Normally you need to be root or have privileges to mount a filesystem. Right? I am aware there is effort going on to allow non-privilege user mount in container. That can open a can of worms if it is not done properly. With privileges, there is a lot of ways one can screw up the system. So I am not less concern about this particular issue. Cheers, Longman