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

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

On 5/29/19 10:47 AM, Dave Chinner wrote:
> On Wed, May 29, 2019 at 10:01:58AM +0800, Shiyang Ruan wrote:
>>
>> 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 ...;
>> }
> 
> Why does this need to be in dax_iomap_rw()?
> 
> We already do post-dax_iomap_rw() "io-end callbacks" directly in
> xfs_file_dax_write() to update the file size....

Yes, but we also need to call ->xfs_reflink_end_cow() after a COW 
operation.  And an is-cow flag(from iomap) is also needed to determine 
if we call it.  I think it would be better to put this into 
->dax_iomap_rw() as a callback function.

So sorry for my poor expression.

> 
> Cheers,
> 
> Dave.
> 

-- 
Thanks,
Shiyang Ruan.


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