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

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.