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

[Shiyang Ruan]

On 2021/8/20 23:18, Dan Williams wrote:
> On Thu, Aug 19, 2021 at 11:13 PM ruansy.fnst <ruansy.fnst@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > > > ---
> > > >    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.

>          bool cow = xfs_is_cow_inode(ip);
>
>          if (cow) {
>                  if (written <= 0)
>                          xfs_reflink_cancel_cow_range(ip, pos, length, true)
>          }

--
Thanks,
Ruan.