Re: [PATCH 1/2] btrfs: volumes: Return the mapped length for discard

[Filipe Manana <fdmanana@gmail.com>]

On Wed, Oct 23, 2019 at 9:53 AM Qu Wenruo <wqu@suse.com> wrote:
>

Hi Qu,

> For btrfs_map_block(), if we pass @op == BTRFS_MAP_DISCARD, the @length
> parameter will not be updated to reflect the real mapped length.
>
> This means for discard operation, we won't know how many bytes are
> really mapped.

Ok, and what's the consequence? The changelog should really say what
is the problem, what the bug is.
The cover letter and the second patch explain what problems are being
solved, but not this change.

>
> Fix this by changing assigning the mapped range length to @length
> pointer, so btrfs_map_block() for BTRFS_MAP_DISCARD also return mapped
> length.
>
> During the change, also do a minor modification to make the length
> calculation a little more straightforward.
> Instead of previously calculated @offset, use "em->end - bytenr" to
> calculate the actually mapped length.

I really don't like much mixing a cleanup with a fix. I would prefer
two separate patches, no matter how small or trivial it is.
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Other than that, it looks good to me.
Thanks.

> ---
>  fs/btrfs/volumes.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index cdd7af424033..f66bd0d03f44 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5578,12 +5578,13 @@ void btrfs_put_bbio(struct btrfs_bio *bbio)
>   * replace.
>   */
>  static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
> -                                        u64 logical, u64 length,
> +                                        u64 logical, u64 *length_ret,
>                                          struct btrfs_bio **bbio_ret)
>  {
>         struct extent_map *em;
>         struct map_lookup *map;
>         struct btrfs_bio *bbio;
> +       u64 length = *length_ret;
>         u64 offset;
>         u64 stripe_nr;
>         u64 stripe_nr_end;
> @@ -5616,7 +5617,8 @@ static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
>         }
>
>         offset = logical - em->start;
> -       length = min_t(u64, em->len - offset, length);
> +       length = min_t(u64, em->start + em->len - logical, length);
> +       *length_ret = length;
>
>         stripe_len = map->stripe_len;
>         /*
> @@ -6031,7 +6033,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
>
>         if (op == BTRFS_MAP_DISCARD)
>                 return __btrfs_map_block_for_discard(fs_info, logical,
> -                                                    *length, bbio_ret);
> +                                                    length, bbio_ret);
>
>         ret = btrfs_get_io_geometry(fs_info, op, logical, *length, &geom);
>         if (ret < 0)
> --
> 2.23.0
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”