On Fri, Apr 15, 2016 at 05:08:22PM +0800, Qu Wenruo wrote:
> Current btrfs qgroup design implies a requirement that after calling
> btrfs_qgroup_account_extents() there must be a commit root switch.
>
> Normally this is OK, as btrfs_qgroup_accounting_extents() is only called
> inside btrfs_commit_transaction() just be commit_cowonly_roots().
>
> However there is a exception at create_pending_snapshot(), which will
> call btrfs_qgroup_account_extents() but no any commit root switch.
>
> In case of creating a snapshot whose parent root is itself (create a
> snapshot of fs tree), it will corrupt qgroup by the following trace:
> (skipped unrelated data)
> ======
> btrfs_qgroup_account_extent: bytenr = 29786112, num_bytes = 16384,
> nr_old_roots = 0, nr_new_roots = 1
> qgroup_update_counters: qgid = 5, cur_old_count = 0, cur_new_count = 1, rfer
> = 0, excl = 0
> qgroup_update_counters: qgid = 5, cur_old_count = 0, cur_new_count = 1, rfer
> = 16384, excl = 16384
> btrfs_qgroup_account_extent: bytenr = 29786112, num_bytes = 16384,
> nr_old_roots = 0, nr_new_roots = 0
> ======
>
> The problem here is in first qgroup_account_extent(), the
> nr_new_roots of the extent is 1, which means its reference got
> increased, and qgroup increased its rfer and excl.
>
> But at second qgroup_account_extent(), its reference got decreased, but
> between these two qgroup_account_extent(), there is no switch roots.
> This leads to the same nr_old_roots, and this extent just got ignored by
> qgroup, which means this extent is wrongly accounted.
>
> Fix it by call commit_cowonly_roots() after qgroup_account_extent() in
> create_pending_snapshot(), with needed preparation.
>
> Reported-by: Mark Fasheh <mfas...@suse.de>
> Signed-off-by: Qu Wenruo <quwen...@cn.fujitsu.com>
Ok, this version seems to be giving me the right numbers. I'll send a test
for it shortly. I'd still like to know if this patch introduces an
unintended side effects but otherwise, thanks Qu.
--Mark
--
Mark Fasheh
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