Re: [PATCH v6 2/5] fs: Convert kiocb rw_hint from enum to u16

[Adam Manzanares]

On 5/22/18 9:30 AM, Jens Axboe wrote:
> On 5/22/18 10:24 AM, Goldwyn Rodrigues wrote:
>>
>>
>> On 05/22/2018 10:32 AM, Jens Axboe wrote:
>>> On 5/22/18 9:07 AM, adam.manzanares@xxxxxxx wrote:
>>>> From: Adam Manzanares <adam.manzanares@xxxxxxx>
>>>>
>>>> In order to avoid kiocb bloat for per command iopriority support, rw_hint
>>>> is converted from enum to a u16. Added a guard around ki_hint assignment.
>>>>
>>>> Signed-off-by: Adam Manzanares <adam.manzanares@xxxxxxx>
>>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>>>> ---
>>>>   include/linux/fs.h | 13 +++++++++++--
>>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>>> index 7f07977bdfd7..50de40dbbb85 100644
>>>> --- a/include/linux/fs.h
>>>> +++ b/include/linux/fs.h
>>>> @@ -284,6 +284,8 @@ enum rw_hint {
>>>>   	WRITE_LIFE_EXTREME	= RWH_WRITE_LIFE_EXTREME,
>>>>   };
>>>>   
>>>> +#define MAX_KI_HINT		((1 << 16) - 1) /* ki_hint type is u16 */
>>>
>>> Instead of having to do this and now rely on those now being synced,
>>> how about something like the below.
>>>
>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>> index 760d8da1b6c7..070438d0b62d 100644
>>> --- a/include/linux/fs.h
>>> +++ b/include/linux/fs.h
>>> @@ -299,7 +299,7 @@ struct kiocb {
>>>   	void (*ki_complete)(struct kiocb *iocb, long ret, long ret2);
>>>   	void			*private;
>>>   	int			ki_flags;
>>> -	enum rw_hint		ki_hint;
>>> +	u16			ki_hint;
>>>   } __randomize_layout;
>>>   
>>>   static inline bool is_sync_kiocb(struct kiocb *kiocb)
>>> @@ -1927,12 +1927,22 @@ static inline enum rw_hint file_write_hint(struct file *file)
>>>   
>>>   static inline int iocb_flags(struct file *file);
>>>   
>>> +static inline u16 ki_hint_validate(enum rw_hint hint)
>>> +{
>>> +	typeof(((struct kiocb *)0)->ki_hint) max_hint = -1;
>>
>> This looks complex to me. Would force a reader to lookback at what
>> datatype ki_hint is. I'd prefer to declare it as u16 max_hint = -1, or
>> even the previous #define MAX_KI_HINT format is easier to read. Just a
>> program reading style you are comfortable with though.
> 
> How is it complex? It's defining a type that'll be the same as ki_hint
> in the kiocb, which is _exactly_ what we care about. Any sort of other
> definition will rely on those two locations now being synced. The
> above will never break.
> 
> So I strongly disagree. The above will _never_ require the reader to
> figure out what the type is. Any other variant will _always_ require
> the reader to check if they are the same.
> 

I do think the previous code was a bit easier to parse at first glance, 
but that is outweighed by the fact that the validate function is now 
directly tied to the kiocb ki_hint type.

I also missed one spot where I should have used ki_hint_validate. Will 
resend soon.