Re: [PATCH V5] Btrfs: snapshot-aware defrag

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On Wed, Jan 23, 2013 at 1:51 AM, Liu Bo <bo.li.liu@xxxxxxxxxx> wrote:
> On Tue, Jan 22, 2013 at 11:41:19AM -0600, Mitch Harder wrote:
>> On Thu, Jan 17, 2013 at 8:42 AM, Mitch Harder
>> <mitch.harder@xxxxxxxxxxxxxxxx> wrote:
>> > On Wed, Jan 16, 2013 at 6:36 AM, Liu Bo <bo.li.liu@xxxxxxxxxx> wrote:
>> >> This comes from one of btrfs's project ideas,
>> >> As we defragment files, we break any sharing from other snapshots.
>> >> The balancing code will preserve the sharing, and defrag needs to grow this
>> >> as well.
> [...]
>> >
>> > I've been testing this patch on a 3.7.2 kernel merged with the
>> > for-linus branch for the 3.8_rc kernels, and I'm seeing the following
>> > error:
>> >
>>
>> I've reproduced the error with CONFIG_DEBUG_LIST enabled, which shows
>> some problem with an entry in the list.
>>
>> [59312.260441] ------------[ cut here ]------------
>> [59312.260454] WARNING: at lib/list_debug.c:62 __list_del_entry+0x8d/0x98()
>> [59312.260458] Hardware name: OptiPlex 745
>> [59312.260461] list_del corruption. next->prev should be
>> ffff88006511c438, but was dead000000200200
>
> LIST_POISON2 -> (000000200200)
> So we can know that the next one is deleted from the list even _earlier_
> than the current one is.
>
> Any other messages before this warning complains?
>

Just some normal feedback from a metadata balance I had run.

[14057.193343] device fsid 28c688c5-7dbd-4071-b271-1bf6726d8835 devid
1 transid 4 /dev/sda7
[14057.194438] btrfs: force lzo compression
[14057.194446] btrfs: enabling auto defrag
[14057.194449] btrfs: disk space caching is enabled
[14057.194452] btrfs flagging fs with big metadata feature
[14057.194455] btrfs: lzo incompat flag set.
[57508.799193] btrfs: relocating block group 14516486144 flags 4
[57632.178797] btrfs: found 6775 extents
[57633.214701] btrfs: relocating block group 11832131584 flags 4
[57776.400102] btrfs: found 6480 extents
[57777.021175] btrfs: relocating block group 10489954304 flags 4
[57949.182725] btrfs: found 6681 extents
[59312.260441] ------------[ cut here ]------------
[59312.260454] WARNING: at lib/list_debug.c:62 __list_del_entry+0x8d/0x98()
[59312.260458] Hardware name: OptiPlex 745
...

I'm going to try to wrap some debugging around the section of code in
btrfs_clean_old_snapshots() where the dead_roots list is spliced onto
the root list being processed.  The double entry may be slipping in
here.

1764         spin_lock(&fs_info->trans_lock);
1765         list_splice_init(&fs_info->dead_roots, &list);
1766         spin_unlock(&fs_info->trans_lock);
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