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

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 ...; }