Re: [PATCH] [Target_Core_Mod/pSCSI]: Add optional legacy scsi_execute_async() usage for Linux/SCSI passthrough

[Boaz Harrosh <bharrosh@panasas.com>]

On 04/22/2009 10:06 PM, Douglas Gilbert wrote:
> Boaz Harrosh wrote:
>> On 04/21/2009 03:15 PM, Vladislav Bolkhovitin wrote:
>>> Boaz Harrosh, on 04/19/2009 02:56 PM wrote:
>>>> On 04/17/2009 04:09 AM, Nicholas A. Bellinger wrote:
>>> [..]
>>>
>>>> Are you aware that scsi_execute_async() has gone in 2.6.30-rc1?
>>>>
>>>> I'm not sure what would be the best alternative for you. I would say
>>>> a bio, but it is still being debated. Your current options are:
>>>>
>>>> 1. bio_alloc then loop () bio_add_pc_page, and finally blk_rq_append_bio
>>>>    (Which block people don't like)
>>>> 2. sglist => page-pointers-array translation and blk_rq_map_user with 
>>>>    struct rq_map_data mode. (not possible with all kind of sglists)
>>>> 2. sglist => iovec translation and blk_rq_map_user_iov()
>>>>    (Very very ugly mapping of pages to virtual pointers)
>>>>
>>>> I have a similar situation with my OSD code.
>>> Do you have somewhere in it a need to run an arbitrary CDB with data 
>>> pages stored in an sglist? Is that code accepted in the mainline?
>>>
>> No, I have a direct bio which comes from two sources.
>> 1. A bio prepared by a filesystem to describe a write/read to a file (osd object)
>> 2. A cloned bio that comes from a stacking block-device over osd-object.
>>
>> So I do not have an sglist at all, anywhere in code.
>>
>>> If yes, why not to resurrect the necessary bits of scsi_execute_async() 
>>> (option (1) above)? It was deleted, because in 2.6.30 there are no users 
>>> of it left, but if there are users (OSD), then why not to return it? 
>>> Seems nothing better for the sg->bio case can be invented.
>>>
>> I hate scsi_execute_async() for lots of reasons, 
>> 1. The cover-up of an historical abuse of sglists by sg/sr
>> 2. Override of use_sg/data-pointer crap
> 
> Hate, abuse, crap??
> [I believe the author of scsi_execute_async() is cc-ed on
> your post.]
> 

Sorry, no offence intended. I meant today in the light of
better options.

> It served a purpose when the block layer was taking over
> sglist handling and was synchronous. Block error handling
> was non-existent or poor at the time. The block layer
> wanted to disown the sg and st drivers because they didn't
> fit its crude model. So scsi_execute_async() was our
> "get out of the !@#$ing block system" card.
> 

I totally agree. At the time it was made it was the most
advance IO concept in the kernel. And it served as an example
of what should be done. Look how long it took to replace it.

> Happily the block system has evolved to being able to
> provide the async handling and error processing that
> it, sg and st need. It is now a non-block system.
> 
> So scsi_execute_async() has become superfluous.
> 

There was a time around 2.6.17 or so, when it was made
to go over block-layer, and it became just a translation from
sglist to a bio. That was the time when it started to be a layering
violation and started to hurt. But it took so long to clean all
users and put things in order.

So now when people start missing it again, I use harsh words, just to
make a technical stand. I never meant it in the historical sense, at
the time and context it was written it was brilliant.

>> 3. What is data-format got to do with sync/async execution
> 
> A fair bit in async. For example I'd like to know when data-out
> buffers can be re-used and data-in buffers are ready. If it was
> really clever it could alert me when thresholds were met in
> the data-in buffer.
> 

I do that today, have 3-4 outstanding async SG_IO in flight before
starting to wait on the first buffers fifo-style, Good sized requests
give you very good balance, of preparing the next buffers while previous
buffers are doing IO. So the disk is kept busy and all processing and
io preparation is done in parallel. I found that calculating sizes in
time units gives best results. Like 1/3 second for a cycle, that is 100-80
milliseconds per request.

> Doug Gilbert
> 
>> 4. the need of all that scsi_io_context and the re-invention
>>    of async_done API.
>> 5. ...
>> Good riddence
>>
>> But you might be looking into an API introduced by an RFC by
>> Tejun Heo in the form of blk_rq_map_kern_sgl() which should
>> be a more appropriate general API for your use.
>> (And is not directly usable for me in OSD)
>>
>>> Vlad
>> Boaz
>>
> 

Sorry, I have the out most respect for you
Boaz