Re: [PATCH 12/15] vfs: implement opportunistic short dedupe

[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>
>
> For a given dedupe request, the bytes_deduped field in the control
> structure tells userspace if we managed to deduplicate some, but not all
> of, the requested regions starting from the file offsets supplied.
> However, due to sloppy coding, the current dedupe code returns
> FILE_DEDUPE_RANGE_DIFFERS if any part of the range is different.
> Fix this so that we can actually support partial request completion.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/read_write.c    |   44 +++++++++++++++++++++++++++++++++++---------
>  include/linux/fs.h |    2 +-
>  2 files changed, 36 insertions(+), 10 deletions(-)
>
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 292d68c2f47c..9be9f261edd2 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1781,13 +1781,11 @@ int vfs_clone_file_prep(struct file *file_in, loff_t pos_in,
>          * Check that the extents are the same.
>          */
>         if (is_dedupe) {
> -               bool            is_same = false;
> -
>                 ret = vfs_dedupe_file_range_compare(inode_in, pos_in,
> -                               inode_out, pos_out, *len, &is_same);
> +                               inode_out, pos_out, len);
>                 if (ret)
>                         return ret;
> -               if (!is_same)
> +               if (*len == 0)
>                         return -EBADE;
>         }
>
> @@ -1872,13 +1870,30 @@ static struct page *vfs_dedupe_get_page(struct inode *inode, loff_t offset)
>         return page;
>  }
>
> +static unsigned int vfs_dedupe_memcmp(const char *s1, const char *s2,
> +                                     unsigned int cmp_len)
> +{
> +       const char *orig_s1 = s1;
> +       const char *e1 = s1 + cmp_len;
> +       const char *e2 = s2 + cmp_len;
> +
> +       while (s1 < e1 && s2 < e2) {
> +               if (*s1 != *s2)
> +                       break;
> +               s1++;
> +               s2++;
> +       }
> +
> +       return s1 - orig_s1;
> +}
> +

A few nits:
'len' wouldn't have been ambiguous in this context.
I find the for loop in memcmp more elegant. It is definitely shorter.
Not sure how differently the variants compile, but decrementing
count/len seems much more sane then checking 2 conditions that
always have the same result.

Thanks,
Amir.