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

[Dave Chinner <david@fromorbit.com>]

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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com