Re: [uml-devel] btrfs failure and BUG_ON behaviour

[richard -r. w. <ric...@gm...>]

On Sat, Apr 9, 2011 at 4:04 PM, Sergei Trofimovich <sl...@gm...> wrote:
> I've decided to use UML when caught some hard to
> debug OOpses on btrfs.
>
> The first attempt to use btrfs on UML gave me stable
> UML crash. I suspect it's a UML's problem.
>
> So I have some questions here (I'm on x86_64,
> 2.6.39-rc2):
>
> 1. (major one) BUG_ON trace in UML does not look
>   as in real kernel. ud2 handler does not show us nice
>   backtrace, but calls some suspicious handler.
>
> /mnt/btr # touch asd-`seq 1 10000`
> [   95.540000] Kernel panic - not syncing: Kernel mode signal 4
> [   95.540000] Call Trace:
> [   95.540000] 602d9908:  [<60227a48>] panic+0xea/0x1e0
> [   95.540000] 602d99b8:  [<600342ed>] do_softirq+0x4d/0x70
> [   95.540000] 602d9a08:  [<60016947>] relay_signal+0x87/0xa0
> [   95.540000] 602d9a18:  [<60021f70>] set_signals+0x30/0x40
> [   95.540000] 602d9a38:  [<60021df9>] sig_handler_common+0x59/0xd0
> [   95.540000] 602d9a58:  [<60021f2c>] real_alarm_handler+0x3c/0x50
> [   95.540000] 602d9ae0:  [<6018c0a0>] strncpy+0x20/0x30
> [   95.540000] 602d9b78:  [<6002200f>] sig_handler+0x3f/0x50
> [   95.540000] 602d9b98:  [<600222a1>] handle_signal+0x71/0xb0
> [   95.540000] 602d9be8:  [<60023630>] hard_handler+0x10/0x20
> [   95.540000] 602d9ca8:  [<6011263c>] lookup_inline_extent_backref+0x2dc/0x3f0
> [   95.540000]
> [   95.540000]
> [   95.540000] Pid: 1, comm: sh Not tainted 2.6.39-rc2+
>
>   Here we have BUG_ON in lookup_inline_extent_backref+0x2dc/0x3f0, but that pesky
>   hard_handler. Is it an UML bug or known and expected behaviour?

Can you please test the attached patch?
The trace was broken long before I started working on UML.

>   Now I use 'gdb -p $pid' to get nicer backtrace:
>
> Program received signal SIGILL, Illegal instruction.
> lookup_inline_extent_backref (trans=0x70c6e000, root=<value optimized out>, path=0x70c82000,
>    ref_ret=<value optimized out>, bytenr=<value optimized out>, num_bytes=<value optimized out>, parent=0,
>    root_objectid=5, owner=0, offset=0, insert=0) at /home/slyfox/linux-2.6/fs/btrfs/extent-tree.c:1399
> 1399            BUG_ON(ret);
>
>   Can UML call the same ud2 handler as the real handler? We would see something like:
>
> [  155.038388] kernel BUG at /mnt/archive/src/linux-2.6/fs/btrfs/inode.c:2967!
> [  155.038643] invalid opcode: 0000 [#1] PREEMPT SMP
>
>   How does UML implement running userspace processes and kernel threads in it?
>   I guess it's a problem, why UML can't distinct where ud2 comes from and call
>   proper handler.
>
> 2. btrfs in UML does not work for me at all. It corrupts data right off
>   the start. I only need to format the filesystem, touch there ~20
>   files and I get corrupted FS:
>
>   I run the following tiny script in UML:
>
> /tools/mkfs.btrfs /dev/ubda
> mount /mnt/btr/
> cd /mnt/btr/
> touch `seq 1 11`
> ls
>
>   The result is:
> / # ./kill_btr
>
> WARNING! - Btrfs v0.19-35-g1b444cd-dirty IS EXPERIMENTAL
> WARNING! - see http://btrfs.wiki.kernel.org before using
>
> fs created label (null) on /dev/ubda
>        nodesize 4096 leafsize 4096 sectorsize 4096 size 1.91GB
> Btrfs v0.19-35-g1b444cd-dirty
> [    2.770000] device fsid f2407093ae1f3a78-9e203194e541bfbd devid 1 transid 7 /dev/ubda
> [    2.770000] btrfS: invalid dir item name len: 12594
> [    2.770000] btrfS: invalid dir item name len: 0
> [    2.770000] btrfS: invalid dir item name len: 0
> 11
>
> Corruption. Only last file is seen. Later, after unmount I get an OOps.
>
> It does not happen in real OS. I've enabled all the corruption
> detecting tehniques in the kernel config and it didn't change picture.
> I can share .config if you like. kernel builds only 4 minutes on my slow
> laptop.
>
> Is there serious problems in UML block device handling?
>
> I run UML this way:
> ./vmlinux                                        \
>    ubd0=$(pwd)/btr.img                          \
>    root=/dev/root                               \
>    rootflags="$(pwd)/root" rw rootfstype=hostfs \
>    mem=256M init=/init                          \
>
> btr.img is a 2048000000 bytes sized file. rootfs contains statically linked
> busybox and latest statically linked btrfs-progs. I can share my root if
> you are interested. It's tiny (btrfs-progs tree is mounted from hostfs):
> $ find root/ -type f
> root/bin/busybox
> root/bin/strace
> root/kill_btr
> root/etc/fstab
> root/init
>
> Thank you!
>
> --
>
>  Sergei
>
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>



-- 
Thanks,
//richard