Messages in this thread | | | From | Naohiro Aota <> | Subject | Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs | Date | Mon, 5 Sep 2016 04:32:37 +0000 |
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2016-09-02 (金) の 09:35 -0400 に Josef Bacik さんは書きました: > 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.
Regards, Naohiro
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