Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap_ops for end of reflink

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Aug 26, 2021 at 8:30 PM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>
>
>
> On 2021/8/20 23:18, Dan Williams wrote:
> > On Thu, Aug 19, 2021 at 11:13 PM ruansy.fnst <ruansy.fnst@fujitsu.com> wrote:
> >>
> >>
> >>
> >> On 2021/8/20 上午11:01, Dan Williams wrote:
> >>> On Sun, Aug 15, 2021 at 11:05 PM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
> >>>>
> >>>> After writing data, reflink requires end operations to remap those new
> >>>> allocated extents.  The current ->iomap_end() ignores the error code
> >>>> returned from ->actor(), so we introduce this dax_iomap_ops and change
> >>>> the dax_iomap_*() interfaces to do this job.
> >>>>
> >>>> - the dax_iomap_ops contains the original struct iomap_ops and fsdax
> >>>>       specific ->actor_end(), which is for the end operations of reflink
> >>>> - also introduce dax specific zero_range, truncate_page
> >>>> - create new dax_iomap_ops for ext2 and ext4
> >>>>
> >>>> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> >>>> ---
> >>>>    fs/dax.c               | 68 +++++++++++++++++++++++++++++++++++++-----
> >>>>    fs/ext2/ext2.h         |  3 ++
> >>>>    fs/ext2/file.c         |  6 ++--
> >>>>    fs/ext2/inode.c        | 11 +++++--
> >>>>    fs/ext4/ext4.h         |  3 ++
> >>>>    fs/ext4/file.c         |  6 ++--
> >>>>    fs/ext4/inode.c        | 13 ++++++--
> >>>>    fs/iomap/buffered-io.c |  3 +-
> >>>>    fs/xfs/xfs_bmap_util.c |  3 +-
> >>>>    fs/xfs/xfs_file.c      |  8 ++---
> >>>>    fs/xfs/xfs_iomap.c     | 36 +++++++++++++++++++++-
> >>>>    fs/xfs/xfs_iomap.h     | 33 ++++++++++++++++++++
> >>>>    fs/xfs/xfs_iops.c      |  7 ++---
> >>>>    fs/xfs/xfs_reflink.c   |  3 +-
> >>>>    include/linux/dax.h    | 21 ++++++++++---
> >>>>    include/linux/iomap.h  |  1 +
> >>>>    16 files changed, 189 insertions(+), 36 deletions(-)
> >>>>
> >>>> diff --git a/fs/dax.c b/fs/dax.c
> >>>> index 74dd918cff1f..0e0536765a7e 100644
> >>>> --- a/fs/dax.c
> >>>> +++ b/fs/dax.c
> >>>> @@ -1348,11 +1348,30 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
> >>>>           return done ? done : ret;
> >>>>    }
> >>>>
> >>>> +static inline int
> >>>> +__dax_iomap_iter(struct iomap_iter *iter, const struct dax_iomap_ops *ops)
> >>>> +{
> >>>> +       int ret;
> >>>> +
> >>>> +       /*
> >>>> +        * Call dax_iomap_ops->actor_end() before iomap_ops->iomap_end() in
> >>>> +        * each iteration.
> >>>> +        */
> >>>> +       if (iter->iomap.length && ops->actor_end) {
> >>>> +               ret = ops->actor_end(iter->inode, iter->pos, iter->len,
> >>>> +                                    iter->processed);
> >>>> +               if (ret < 0)
> >>>> +                       return ret;
> >>>> +       }
> >>>> +
> >>>> +       return iomap_iter(iter, &ops->iomap_ops);
> >>>
> >>> This reorganization looks needlessly noisy. Why not require the
> >>> iomap_end operation to perform the actor_end work. I.e. why can't
> >>> xfs_dax_write_iomap_actor_end() just be the passed in iomap_end? I am
> >>> not seeing where the ->iomap_end() result is ignored?
> >>>
> >>
> >> The V6 patch[1] was did in this way.
> >> [1]https://lore.kernel.org/linux-xfs/20210526005159.GF202144@locust/T/#m79a66a928da2d089e2458c1a97c0516dbfde2f7f
> >>
> >> But Darrick reminded me that ->iomap_end() will always take zero or
> >> positive 'written' because iomap_apply() handles this argument.
> >>
> >> ```
> >>          if (ops->iomap_end) {
> >>                  ret = ops->iomap_end(inode, pos, length,
> >>                                       written > 0 ? written : 0,
> >>                                       flags, &iomap);
> >>          }
> >> ```
> >>
> >> So, we cannot get actual return code from CoW in ->actor(), and as a
> >> result, we cannot handle the xfs end_cow correctly in ->iomap_end().
> >> That's where the result of CoW was ignored.
> >
> > Ah, thank you for the explanation.
> >
> > However, this still seems like too much code thrash just to get back
> > to the original value of iter->processed. I notice you are talking
> > about iomap_apply(), but that routine is now gone in Darrick's latest
> > iomap-for-next branch. Instead iomap_iter() does this:
> >
> >          if (iter->iomap.length && ops->iomap_end) {
> >                  ret = ops->iomap_end(iter->inode, iter->pos, iomap_length(iter),
> >                                  iter->processed > 0 ? iter->processed : 0,
>
> As you can see, here is the same logic as the old iomap_apply(): the
> negative iter->processed won't be passed into ->iomap_end().
>
> >                                  iter->flags, &iter->iomap);
> >                  if (ret < 0 && !iter->processed)
> >                          return ret;
> >          }
> >
> >
> > I notice that the @iomap argument to ->iomap_end() is reliably coming
> > from @iter. So you could do the following in your iomap_end()
> > callback:
> >
> >          struct iomap_iter *iter = container_of(iomap, typeof(*iter), iomap);
> >          struct xfs_inode *ip = XFS_I(inode);
> >          ssize_t written = iter->processed;
>
> The written will be 0 or positive.  The original error code is ingnored.

Correct, but you can use container_of() to get back to the iter and
consider the raw untranslated value of iter->processed. As Christoph
mentioned this needs a comment explaining the layering violation, but
that's a cleaner change than the dax_iomap_ops approach.