Re: [PATCH 08/15] vfs: change clone and dedupe range function pointers to return bytes completed

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Oct 5, 2018 at 3:46 AM Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> Change the clone_file_range and dedupe_file_range functions to return
> the number of bytes they operated on.  This is the precursor to allowing
> fs implementations to return short clone/dedupe results to the user,
> which will enable us to obey resource limits in a graceful manner.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

[...]

> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index aeaefd2a551b..6d792d817538 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -487,16 +487,21 @@ static ssize_t ovl_copy_file_range(struct file *file_in, loff_t pos_in,
>                             OVL_COPY);
>  }
>
> -static int ovl_clone_file_range(struct file *file_in, loff_t pos_in,
> +static s64 ovl_clone_file_range(struct file *file_in, loff_t pos_in,
>                                 struct file *file_out, loff_t pos_out, u64 len)
>  {
> -       return ovl_copyfile(file_in, pos_in, file_out, pos_out, len, 0,
> -                           OVL_CLONE);
> +       int ret;
> +
> +       ret = ovl_copyfile(file_in, pos_in, file_out, pos_out, len, 0,
> +                          OVL_CLONE);
> +       return ret < 0 ? ret : len;
>  }
>
> -static int ovl_dedupe_file_range(struct file *file_in, loff_t pos_in,
> +static s64 ovl_dedupe_file_range(struct file *file_in, loff_t pos_in,
>                                  struct file *file_out, loff_t pos_out, u64 len)
>  {
> +       int ret;
> +
>         /*
>          * Don't copy up because of a dedupe request, this wouldn't make sense
>          * most of the time (data would be duplicated instead of deduplicated).
> @@ -505,8 +510,9 @@ static int ovl_dedupe_file_range(struct file *file_in, loff_t pos_in,
>             !ovl_inode_upper(file_inode(file_out)))
>                 return -EPERM;
>
> -       return ovl_copyfile(file_in, pos_in, file_out, pos_out, len, 0,
> -                           OVL_DEDUPE);
> +       ret = ovl_copyfile(file_in, pos_in, file_out, pos_out, len, 0,
> +                          OVL_DEDUPE);
> +       return ret < 0 ? ret : len;
>  }
>

This is not pretty at all.
You are blocking the propagation of partial dedupe/clone result
of files that are accessed via overlay over xfs.

Please extend the interface change to the vfs helpers
(i.e. vfs_clone_file_range()) and then the change above is not needed.

Of course you would need to change the 3 callers of
vfs_clone_file_range() that expect 0 is ok.

Please take a look at commit
a725356b6659 ("vfs: swap names of {do,vfs}_clone_file_range()")

That was just merged for rc7.

I do apologize for the churn, but it's a semantic mistake that
I made that needed fixing, so please rebase your work on top
of that and take care not to trip over it.

ioctl_file_clone() and ovl_copy_up_data() just need to interpret
positive return value correctly.
nfsd4_clone_file_range() should have the same return value as
vfs_clone_file_range() to be interpreted in nfsd4_clone(), following
same practice as nfsd4_copy_file_range().

[...]

> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 2a4141d36ebf..e5755340e825 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1759,10 +1759,12 @@ struct file_operations {
>  #endif
>         ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
>                         loff_t, size_t, unsigned int);
> -       int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t,
> -                       u64);
> -       int (*dedupe_file_range)(struct file *, loff_t, struct file *, loff_t,
> -                       u64);
> +       s64 (*clone_file_range)(struct file *file_in, loff_t pos_in,
> +                               struct file *file_out, loff_t pos_out,
> +                               u64 count);
> +       s64 (*dedupe_file_range)(struct file *file_in, loff_t pos_in,
> +                                struct file *file_out, loff_t pos_out,
> +                                u64 count);

Matthew has objected a similar interface change when it was proposed by Miklos:
https://marc.info/?l=linux-fsdevel&m=152570317110292&w=2
https://marc.info/?l=linux-fsdevel&m=152569298704781&w=2

He claimed that the interface should look like this:
+       loff_t (*dedupe_file_range)(struct file *src, loff_t src_off,
+                       struct file *dst, loff_t dst_off, loff_t len);

Thanks,
Amir.