Re: [PATCH RFC 00/14] Qgroup reserved space fixing framework

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Again, later patches are blocked by the Exchange mail server.....

I'll send it again using another mailbox(quwenruo.btrfs@gmx.com).

Thanks,
Qu

Qu Wenruo wrote on 2015/09/01 15:21 +0800:
> !!!!!!WARNING START!!!!!!
> These patch is just a WIP patchset, although it fixed a qgroup reserved
> space leaking bug in normal COW case, it still lacks fix for other
> corner case, like NODATACOW or prealloc case, and a lot of old
> facilities are not cleaned up yet.
>
> The reason to send the WIP patchset is to check if the patchset has some
> deep structure bug, to avoid another rework after the whole patchset is
> finished
> !!!!!!WARNING END!!!!!!
>
> Although we have already reworked btrfs qgroup accounting part in
> v4.2-rc1, but qgroup reserve part still has a problem of leaking
> reserved space.
>
> [[BUG]]
> One of the most common case to trigger the bug is the following method:
> 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.
>
> [[CAUSE]]
> The problem is caused by the fact that qgroup will reserve space even
> the data space is already reserved.
>
> In above reproducer, even 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() follow
> the very bad btrfs space allocating principle:
>    Allocate as much as you needed, even it's not fully used.
>
> So in the patchset, we introduce a lot of facilities:
> 1) Per inode data rsv map
>     Record which range of a file has already been reserved.
>     Dirty range will be released when the range is written into disk.
>     And for any request to reserve space on already reserved range, just
>     skip it to avoid
>
> 2) Delayed ref head qgroup members
>     After a range of data is written into disk, we can't keep the dirty
>     range in data rsv map or just release reserved space.
>
>     If we keep dirty range in data rsv map, next write will consider
>     there is no need to reserve space, but new write will be cowed, and
>     cause another extent to take qgroup space.
>     So if keep dirty range, it'll cause qgroup accounting to exceed
>     limit.
>
>     On the other hand, if just release and free the reserved space, we
>     can still exceed the limit by allowing over-reserve.
>
>     So here, we must only release the range, but keep the reserved space
>     recorded in other place.
>     With the new qgroup accounting framework, only delayed_ref_head is
>     safe and will be run at the same time as btrfs qgroup accounting.
>
> 3) New delalloc_reserve_space/check_data_free_space facilities to
>     support accurate reserve space.
>     Unlike old implement, which consider it enough by only using
>     num_bytes.
>     New facilities all need a exact range [start, start + len) to reserve
>     space.
>
> More detailed info can be found in each commit message and source
> commend.
>
> Qu Wenruo (14):
>    btrfs: qgroup: New function declaration for new reserve implement
>    btrfs: qgroup: Implement data_rsv_map init/free functions
>    btrfs: qgroup: Introduce new function to search most left reserve
>      range
>    btrfs: qgroup: Introduce function to insert non-overlap reserve range
>    btrfs: qgroup: Introduce function to reserve data range per inode
>    btrfs: qgroup: Introduce btrfs_qgroup_reserve_data function
>    btrfs: qgroup: Introduce function to release reserved range
>    btrfs: qgroup: Introduce function to release/free reserved data range
>    btrfs: delayed_ref: Add new function to record reserved space into
>      delayed ref
>    btrfs: delayed_ref: release and free qgroup reserved at proper timing
>    btrfs: qgroup: Introduce new functions to reserve/free metadata
>    btrfs: qgroup: Use new metadata reservation.
>    btrfs: extent-tree: Add new verions of btrfs_check_data_free_space
>    btrfs: Use new check_data_free_space for buffered write
>
>   fs/btrfs/btrfs_inode.h |   6 +
>   fs/btrfs/ctree.h       |   5 +
>   fs/btrfs/delayed-ref.c |  29 +++
>   fs/btrfs/delayed-ref.h |  14 ++
>   fs/btrfs/disk-io.c     |   1 +
>   fs/btrfs/extent-tree.c |  68 +++--
>   fs/btrfs/file.c        |  22 +-
>   fs/btrfs/inode.c       |  20 ++
>   fs/btrfs/qgroup.c      | 658 ++++++++++++++++++++++++++++++++++++++++++++++++-
>   fs/btrfs/qgroup.h      |  21 +-
>   fs/btrfs/transaction.c |  34 +--
>   fs/btrfs/transaction.h |   1 -
>   12 files changed, 820 insertions(+), 59 deletions(-)
>