Messages in this thread | | | From | Stanislav Brabec <> | Subject | Re: loop subsystem corrupted after mounting multiple btrfs sub-volumes | Date | Fri, 26 Feb 2016 21:37:22 +0100 |
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On Feb 26, 2016 at 21:05 Austin S. Hemmelgarn wrote:
> It's kind of interesting, but I can't reproduce _any_ of this behavior > with either ext4 or BTRFS when I manually set up the loop devices and > point mount(8) at those instead of using -o loop on a file. That really > seems to indicate that this is caused by something mount(8) is doing > when it's calling losetup.
Behavior of "-oloop" is more similar to "losetup -f /fs.img"? than to "losetup /dev/loop0 /fs.img".
Anyway, I can reproduce without -oloop: # losetup /dev/loop0 /btrfs.img # mount /dev/loop0 /mnt/1 # grep /mnt /proc/self/mountinfo 107 59 0:59 /d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 /mnt/1 rw,relatime shared:45 - btrfs /dev/loop0 rw,space_cache,subvolid=257,subvol=/d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 # losetup /dev/loop1 /btrfs.img # mount -osubvol=/ /dev/loop1 /mnt/2 # grep /mnt /proc/self/mountinfo 107 59 0:59 /d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 /mnt/1 rw,relatime shared:45 - btrfs /dev/loop1 rw,space_cache,subvolid=257,subvol=/d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 108 59 0:59 / /mnt/2 rw,relatime shared:48 - btrfs /dev/loop1 rw,space_cache,subvolid=5,subvol=/ # uname -a Linux oct 4.4.1-1-default #1 SMP PREEMPT Mon Feb 15 11:03:27 UTC 2016 (6398c2d) x86_64 x86_64 x86_64 GNU/Linux
(Note that the system was freshly rebooted. After other experiments, the second line of mountinfo can be missing completely.)
>> 2) mount(2) called after the reproducer returns OK but does nothing. >> > OK, we've determined that mount(2) is misbehaving. That doesn't change > the fact that mount(8) is triggering this, and therefore should itself > be corrected.
> Assume that mount(2) gets fixed so it doesn't lose it's > mind and /proc/self/mountinfo doesn't change. There will still be > issues resulting from mount(8)'s behavior: > 1. BTRFS will lose it's mind and corrupt data when using a multi-device > filesystem (due to the problems with duplicate FS UUID's). > 2. XFS might have similar issues to 1 when using metadata checksumming, > although it's more likely that it won't allow the second mount to succeed. > 3. Most other filesystems will likely end up corrupting data.
Do I understand, that you are saying:
Yes, mounting multiple loop devices associated with one file is a legitimate use, but mount(8) should never do it, because it has other ugly side effects?
OK, it looks like a next task for mount(8) to fix.
-- Best Regards / S pozdravem,
Stanislav Brabec software developer --------------------------------------------------------------------- SUSE LINUX, s. r. o. e-mail: sbrabec@suse.com Lihovarská 1060/12 tel: +49 911 7405384547 190 00 Praha 9 fax: +420 284 084 001 Czech Republic http://www.suse.cz/ PGP: 830B 40D5 9E05 35D8 5E27 6FA3 717C 209F A04F CD76
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