Re: [RFC PATCH] btrfs: Remove 'objectid' member from struct btrfs_root

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018/8/14 上午1:37, David Sterba wrote:
> On Mon, Aug 06, 2018 at 02:17:54PM +0800, Qu Wenruo wrote:
>>> -	u64 objectid;
>>
>> Off topic crazy idea here.
>>
>> I think it is a little crazy, but it should save a lot of objectid
>> related modification:
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index 118346aceea9..e6d70f2309a3 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -1166,7 +1166,10 @@ struct btrfs_root {
>>
>>         unsigned long state;
>>         struct btrfs_root_item root_item;
>> -       struct btrfs_key root_key;
>> +       union {
>> +               struct btrfs_key root_key;
>> +               u64 objectid;
>> +       };
>>         struct btrfs_fs_info *fs_info;
>>         struct extent_io_tree dirty_log_pages;
>>
>> @@ -1198,7 +1201,6 @@ struct btrfs_root {
>>         int last_log_commit;
>>         pid_t log_start_pid;
>>
>> -       u64 objectid;
>>         u64 last_trans;
>>
>>         u32 type;
>>
>> I'm not sure if this is a really crazy idea or a dirty hack to reduce
>> some modification.
> 
> Be wary of such tricks. This might make you feel good for a moment how
> good your C knowlege is, and also might save a few keystrokes. And a few
> years later this costs somebody a week of debugging a mysterious memory
> corrupption under circumstances not imagined right now.

Yep, that's why I'm calling this a "crazy idea" for the same reason.

Just wondering if there is any better way to do it without using a
trick, like anonymous structure inside btrfs_root?
(Purely to improve my C skill if possible)

Anyway, I'm completely OK with current patch.

Thanks,
Qu

> 
> Try to write understandable code. If there is a reason to use tricks,
> like the struct page has to do for performance reasons, then it must be
> documented and justified.
> 


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