Messages in this thread | | | Subject | Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs | From | Josef Bacik <> | Date | Tue, 6 Sep 2016 08:52:57 -0400 |
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On 09/05/2016 12:32 AM, Naohiro Aota wrote: > 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. >
Ok that's what I was missing, thanks
Reviewed-by: Josef Bacik <jbacik@fb.com>
Josef
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