From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 6C8216B0005 for ; Sat, 11 Jun 2016 16:39:23 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id e189so169007207pfa.2 for ; Sat, 11 Jun 2016 13:39:23 -0700 (PDT) Received: from sender163-mail.zoho.com (sender163-mail.zoho.com. [74.201.84.163]) by mx.google.com with ESMTPS id gm10si6739933pac.65.2016.06.11.13.39.21 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sat, 11 Jun 2016 13:39:22 -0700 (PDT) From: "James Johnston" Subject: Placing swap partition on a loop device hangs the system Date: Sat, 11 Jun 2016 20:39:03 -0000 Message-ID: <0f0a01d1c421$4bab08c0$e3011a40$@codenest.com> MIME-Version: 1.0 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: linux-btrfs@vger.kernel.org, linux-mm@kvack.org Hi, It's well-known that btrfs doesn't directly support swap files. = However, a common workaround I read is to make a loop device from a file on the = btrfs file system, and then put a swap partition on the loop device. See for = example: = https://btrfs.wiki.kernel.org/index.php/FAQ#Does_btrfs_support_swap_files= .3F "A workaround ... is to mount a swap file via a loop device." The Arch Linux wiki page on btrfs makes similar recommendations, and in = fact provides a package to automate the process: = https://github.com/Nefelim4ag/systemd-swap/blob/a36efac996b70e95156c43307= 038b4f4ff2bc0ac/systemd-swap.sh#L78 However, I have not been successful in getting this to work on Ubuntu = Linux. In fact, I cannot get swap on a loop device to work on ext4 either, so I = think it is not strictly a btrfs problem. Maybe a bug with the loop driver, or = swap? However I'm mailing btrfs in addition to linux-mm because some btrfs = users use this technique, and I'd be interested to know how they got it working = and what their configuration & kernel versions are. (Also, loop device has no = maintainer or list.) # Userspace is Ubuntu 16.04, all up-to-date: # Tested on kernel built from mainline v4.7-rc2 (i.e. not distro = kernel): uname -r # Prints: 4.7.0-rc2-af8c34ce # Following commands are run on a VM with 1 GB RAM fallocate --length 3000MiB /swaploop losetup --show -f /swaploop mkswap /dev/loop0 swapon /dev/loop0 # Check for swap presence: free -m # Following will make 8 workers that allocate 256 MB RAM each (i.e. = 2 GB). # Since there is only 1 GB RAM, this is guaranteed to use swap. apt-get install stress stress --vm-keep -m 8 At this point, the system almost completely hangs, and it never comes = back. The hypervisor says one CPU in the dual-CPU VM is maxed out, and the = other one is idle. Disk I/O is absent. [ 240.391459] INFO: task kswapd0:38 blocked for more than 120 seconds. [ 240.394980] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.397361] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.400412] INFO: task jbd2/sda2-8:479 blocked for more than 120 = seconds. [ 240.402877] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.405127] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.408233] INFO: task systemd-udevd:594 blocked for more than 120 = seconds. [ 240.412323] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.414109] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.417490] INFO: task gmain:1177 blocked for more than 120 seconds. [ 240.420323] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.421999] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.425073] INFO: task cron:1158 blocked for more than 120 seconds. [ 240.427170] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.428815] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.431385] INFO: task vmtoolsd:1160 blocked for more than 120 = seconds. [ 240.435214] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.437132] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.440538] INFO: task loop0:1561 blocked for more than 120 seconds. [ 240.443889] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.446787] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.451323] INFO: task stress:1575 blocked for more than 120 seconds. [ 240.453545] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.455214] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.458051] INFO: task stress:1576 blocked for more than 120 seconds. [ 240.460344] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.466992] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. [ 240.479494] INFO: task stress:1577 blocked for more than 120 seconds. [ 240.481504] Not tainted 4.7.0-rc2-af8c34ce #1 [ 240.483145] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" = disables this message. Since apparently this worked in the past (???), I'm guessing this is = some kind of regression. If anyone has a working configuration that passes the = above stress test, I'd be willing to try bisecting (e.g. known-good = distribution version & kernel version). Maybe there is some regression, or a = configuration difference between the distributions? (When building 4.7-rc2, I reused = config from Ubuntu 4.4 kernel.) Or if anyone already knows what the problem might be, I'm all ears. :) Best regards, James Johnston -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org