Re: [PATCH 04/18] dax: Introduce IOMAP_DAX_COW to CoW edges during writes

[Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>]

On 5/28/19 5:17 PM, Jan Kara wrote:
> On Mon 27-05-19 16:25:41, Shiyang Ruan wrote:
>> On 5/23/19 7:51 PM, Goldwyn Rodrigues wrote:
>>>>
>>>> Hi,
>>>>
>>>> I'm working on reflink & dax in XFS, here are some thoughts on this:
>>>>
>>>> As mentioned above: the second iomap's offset and length must match the
>>>> first.  I thought so at the beginning, but later found that the only
>>>> difference between these two iomaps is @addr.  So, what about adding a
>>>> @saddr, which means the source address of COW extent, into the struct iomap.
>>>> The ->iomap_begin() fills @saddr if the extent is COW, and 0 if not.  Then
>>>> handle this @saddr in each ->actor().  No more modifications in other
>>>> functions.
>>>
>>> Yes, I started of with the exact idea before being recommended this by Dave.
>>> I used two fields instead of one namely cow_pos and cow_addr which defined
>>> the source details. I had put it as a iomap flag as opposed to a type
>>> which of course did not appeal well.
>>>
>>> We may want to use iomaps for cases where two inodes are involved.
>>> An example of the other scenario where offset may be different is file
>>> comparison for dedup: vfs_dedup_file_range_compare(). However, it would
>>> need two inodes in iomap as well.
>>>
>> Yes, it is reasonable.  Thanks for your explanation.
>>
>> One more thing RFC:
>> I'd like to add an end-io callback argument in ->dax_iomap_actor() to update
>> the metadata after one whole COW operation is completed.  The end-io can
>> also be called in ->iomap_end().  But one COW operation may call
>> ->iomap_apply() many times, and so does the end-io.  Thus, I think it would
>> be nice to move it to the bottom of ->dax_iomap_actor(), called just once in
>> each COW operation.
> 
> I'm sorry but I don't follow what you suggest. One COW operation is a call
> to dax_iomap_rw(), isn't it? That may call iomap_apply() several times,
> each invocation calls ->iomap_begin(), ->actor() (dax_iomap_actor()),
> ->iomap_end() once. So I don't see a difference between doing something in
> ->actor() and ->iomap_end() (besides the passed arguments but that does not
> seem to be your concern). So what do you exactly want to do?

Hi Jan,

Thanks for pointing out, and I'm sorry for my mistake.  It's 
->dax_iomap_rw(), not ->dax_iomap_actor().

I want to call the callback function at the end of ->dax_iomap_rw().

Like this:
dax_iomap_rw(..., callback) {

     ...
     while (...) {
         iomap_apply(...);
     }

     if (callback != null) {
         callback();
     }
     return ...;
}

> 
> 								Honza
> 

-- 
Thanks,
Shiyang Ruan.