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=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_PASS,USER_AGENT_MUTT 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 B3791C43381 for ; Sun, 17 Feb 2019 23:15:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8DF63206C0 for ; Sun, 17 Feb 2019 23:15:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727281AbfBQXPR (ORCPT ); Sun, 17 Feb 2019 18:15:17 -0500 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:42484 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726283AbfBQXPQ (ORCPT ); Sun, 17 Feb 2019 18:15:16 -0500 Received: by atrey.karlin.mff.cuni.cz (Postfix, from userid 512) id 287268055A; Mon, 18 Feb 2019 00:15:07 +0100 (CET) Date: Mon, 18 Feb 2019 00:15:14 +0100 From: Pavel Machek To: "Richard W.M. Jones" , smcdowell@cloudbd.io Cc: kernel list , Andrew Morton Subject: Re: nbd, nbdkit, loopback mounts and memory management Message-ID: <20190217231514.GA10675@amd> References: <20190215191953.GB17897@amd> <20190217084458.GY12500@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="ew6BAiZeqk4r7MaW" Content-Disposition: inline In-Reply-To: <20190217084458.GY12500@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --ew6BAiZeqk4r7MaW Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hi! > So not to dispute that this could be a bug, but I couldn't cause a > deadlock. I wonder if you can see something wrong with my method? >=20 > *** Set up *** >=20 > - kernel 5.0.0-0.rc3.git0.1.fc30.x86_64 > - nbd-client 3.19-1.fc30 > - nbdkit 1.11.5 (git commit ef9d1978ce28) >=20 > Baremetal machine was booted with mem=3D2G to artificially limit the > RAM. The machine has 64G of swap. >=20 > # free -m > total used free shared buff/cache ava= ilable > Mem: 1806 329 1383 0 93 = 1357 > Swap: 65535 179 65356 >=20 > *** Method *** >=20 > I started nbdkit as a 4G RAM disk: >=20 > ./nbdkit memory size=3D4G >=20 > This is implemented as a sparse array with a 2 level page table, and > should allocate (using malloc) every time a new area of the disk is > written to. Exact implementation is here: > https://github.com/libguestfs/nbdkit/tree/master/common/sparse >=20 > I started nbd-client using the -swap option which uses > mlockall(MCL_FUTURE) to lock the client into RAM. >=20 > nbd-client -b 512 -swap localhost /dev/nbd0 >=20 > I then created a filesystem on the RAM disk, mounted it, and copied a > 3G file into it. I tried this various ways, but the variation I was > eventually happy with was: >=20 > mke2fs /dev/nbd0 > mount /dev/nbd0 /tmp/mnt >=20 > dd if=3D/dev/zero of=3D/tmp/big bs=3D1M count=3D3000 > cp /tmp/big /tmp/mnt/big >=20 > I couldn't get any kind of deadlock or failure in this test. >=20 > (Note that if you repeat the test several times, in theory you could > delete the file and fstrim the filesystem, but when I was testing it > to be sure I unmounted everything and killed and restarted nbdkit > between each test.) This looks like quite a good try. I'd try to use mmap() to dirty memory very quickly. But Shaun reported it happened somehow often for them, so he might have a practical test case... better than my theories :-). Best regards, Pavel --=20 (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo= g.html --ew6BAiZeqk4r7MaW Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAlxp6wEACgkQMOfwapXb+vI0kQCffGxdlWhnyXsROH74HAQZlA1i rksAoLVYaLVxcClBd3Con968UpZcNkSt =RuDA -----END PGP SIGNATURE----- --ew6BAiZeqk4r7MaW--