Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs

[Josef Bacik]

On 09/05/2016 12:32 AM, Naohiro Aota wrote:
> 2016-09-02 (é) ã 09:35 -0400 ã Josef Bacik ãããæãããã:
> > On 09/02/2016 03:46 AM, Naohiro Aota wrote:
> > > Currently, btrfs_relocate_chunk() is removing relocated BG by
> > > itself. But
> > > the work can be done by btrfs_delete_unused_bgs() (and it's better
> > > since it
> > > trim the BG). Let's dedupe the code.
> > >
> > > While btrfs_delete_unused_bgs() is already hitting the relocated
> > > BG, it
> > > skip the BG since the BG has "ro" flag set (to keep balancing BG
> > > intact).
> > > On the other hand, btrfs cannot drop "ro" flag here to prevent
> > > additional
> > > writes. So this patch make use of "removed" flag.
> > > btrfs_delete_unused_bgs() now detect the flag to distinguish
> > > whether a
> > > read-only BG is relocating or not.
> >
> > This seems racey to me.  We remove the last part of the block group,
> > it ends up
> > on the unused_bgs_list, we process this list, see that removed isn't
> > set and we
> > skip it, then later we set removed, but it's too late.  I think the
> > right way is
> > to actually do a transaction, set ->removed, manually add it to the
> > unused_bgs_list if it's not already, then end the transaction.  This
> > way we are
> > guaranteed to have the bg on the list when it is ready to be
> > removed.  This is
> > my analysis after looking at it for 10 seconds after being awake for
> > like 30
> > minutes so if I'm missing something let me know.  Thanks,
>
> I don't think a race will happen. Since we are holding
> delete_unused_bgs_mutex here, btrfs_delte_unused_bgs() checks ->removed
> flag after we unlock the mutex i.e. we setup the flag properly. For a
> case btrfs_delete_usused_bgs() checks the BG before we hold
> delte_unused_bgs_mutex, then that BG is removed by it (if it's empty)
> and btrfs_relocate_chunk() should never see it.

Ok that's what I was missing, thanks

Reviewed-by: Josef Bacik <jbacik@xxxxxx>

Josef