Re: [PATCH v2 07/14] fs: iomap: Sub-extent zeroing

Dave Chinner
2024-03-06T21:14:52Z

On Mon, Mar 04, 2024 at 01:04:21PM +0000, John Garry wrote:
> For FS_XFLAG_FORCEALIGN support, we want to treat any sub-extent IO like
> sub-fsblock DIO, in that we will zero the sub-extent when the mapping is
> unwritten.
>
> This will be important for atomic writes support, in that atomically
> writing over a partially written extent would mean that we would need to
> do the unwritten extent conversion write separately, and the write could
> no longer be atomic.
>
> It is the task of the FS to set iomap.extent_shift per iter to indicate
> sub-extent zeroing required.
>
> Maybe a macro like i_blocksize() should be introduced for extent sizes,
> instead of using extent_shift. It would also eliminate excessive use
> of xfs_get_extss() for XFS in future.
>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
> fs/iomap/direct-io.c | 14 ++++++++------
> include/linux/iomap.h | 1 +
> 2 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index bcd3f8cf5ea4..733f83f839b6 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -277,7 +277,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
> {
> const struct iomap *iomap = &iter->iomap;
> struct inode *inode = iter->inode;
> - unsigned int fs_block_size = i_blocksize(inode), pad;
> + unsigned int zeroing_size, pad;
> loff_t length = iomap_length(iter);
> loff_t pos = iter->pos;
> blk_opf_t bio_opf;
> @@ -288,6 +288,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
> size_t copied = 0;
> size_t orig_count;
>
> + zeroing_size = i_blocksize(inode) << iomap->extent_shift;

The iomap interfaces use units of bytes for offsets, sizes, ranges,
etc. Using shifts to define a granularity value seems like a
throwback to decades old XFS code and just a bit weird nowdays. Can
we just pass this as a byte count? i.e.:

zeroing_size = i_blocksize(inode);
if (iomap->extent_size)
zeroing_size = iomap->extent_size;

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com