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

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Jan 23, 2013 at 10:05:04AM -0600, Mitch Harder wrote:
> On Wed, Jan 23, 2013 at 1:51 AM, Liu Bo <bo.li.liu@oracle.com> 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@sabayonlinux.org> wrote:
> >> > On Wed, Jan 16, 2013 at 6:36 AM, Liu Bo <bo.li.liu@oracle.com> 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.

Well, these do fit my expectation, since balance also involves with playing with
root_list, which may lead to the bad situation.

> 
> [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);

hmm, I don't think there is anything wrong in this code.  But you can
give it a shot anyway :)

thanks,
liubo