Re: [PATCH v2 00/23] Rework btrfs qgroup reserved space framework

[Josef Bacik <jbacik@fb.com>]

On 10/08/2015 07:11 PM, Qu Wenruo wrote:
> In previous rework of qgroup, we succeeded in fixing qgroup accounting
> part, making the rfer/excl numbers accurate.
>
> But that's just part of qgroup work, another part of qgroup still has
> quite a lot problem, that's qgroup reserve space part which will lead to
> EQUOT even we are far from the limit.
>
> [[BUG]]
> The easiest way to trigger the bug is,
> 1) Enable quota
> 2) Limit excl of qgroup 5 to 16M
> 3) Write [0,2M) of a file inside subvol 5 10 times without sync
>
> EQUOT will be triggered at about the 8th write.
> But after remount, we can still write until about 15M.
>
> [[CAUSE]]
> The problem is caused by the fact that qgroup will reserve space even
> the data space is already reserved.
>
> In above reproducer, each time we buffered write [0,2M) qgroup will
> reserve 2M space, but in fact, at the 1st time, we have already reserved
> 2M and from then on, we don't need to reserved any data space as we are
> only writing [0,2M).
>
> Also, the reserved space will only be freed *ONCE* when its backref is
> run at commit_transaction() time.
>
> That's causing the reserved space leaking.
>
> [[FIX]]
> The fix is not a simple one, as currently btrfs_qgroup_reserve() will
> allocate whatever caller asked for.
>
> So for accurate qgroup reserve, we introduce a completely new framework
> for data and metadata.
> 1) Per-inode data reserve map
>     Now, each inode will have a data reserve map, recording which range
>     of data is already reserved.
>     If we are writing a range which is already reserved, we won't need to
>     reserve space again.
>
>     Also, for the fact that qgroup is only accounted at commit_trans(),
>     for data commit into disc and its metadata is also inserted into
>     current tree, we should free the data reserved range, but still keep
>     the reserved space until commit_trans().
>
>     So delayed_ref_head will have new members to record how much space is
>     reserved and free them at commit_trans() time.

This is already handled by setting DELALLOC in the io_tree, we do 
similar sort of stuff for the normal enospc accounting, why not use 
that?  Thanks,

Josef