Re: [PATCHv7 5/6] iomap: Allocate iop in ->write_begin() early

Darrick J. Wong
2023-06-05T22:58:38Z

On Mon, Jun 05, 2023 at 04:25:05PM +0530, Ritesh Harjani (IBM) wrote:
> We dont need to allocate an iop in ->write_begin() for writes where the
> position and length completely overlap with the given folio.
> Therefore, such cases are skipped.
>
> Currently when the folio is uptodate, we only allocate iop at writeback
> time (in iomap_writepage_map()). This is ok until now, but when we are
> going to add support for per-block dirty state bitmap in iop, this
> could cause some performance degradation. The reason is that if we don't
> allocate iop during ->write_begin(), then we will never mark the
> necessary dirty bits in ->write_end() call. And we will have to mark all
> the bits as dirty at the writeback time, that could cause the same write
> amplification and performance problems as it is now.
>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Makes sense to me, but moving on to the next patch...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/iomap/buffered-io.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f55a339f99ec..2a97d73edb96 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -571,15 +571,24 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
> size_t from = offset_in_folio(folio, pos), to = from + len;
> size_t poff, plen;
>
> - if (folio_test_uptodate(folio))
> + /*
> + * If the write completely overlaps the current folio, then
> + * entire folio will be dirtied so there is no need for
> + * per-block state tracking structures to be attached to this folio.
> + */
> + if (pos <= folio_pos(folio) &&
> + pos + len >= folio_pos(folio) + folio_size(folio))
> return 0;
> - folio_clear_error(folio);
>
> iop = iomap_iop_alloc(iter->inode, folio, iter->flags);
>
> if ((iter->flags & IOMAP_NOWAIT) && !iop && nr_blocks > 1)
> return -EAGAIN;
>
> + if (folio_test_uptodate(folio))
> + return 0;
> + folio_clear_error(folio);
> +
> do {
> iomap_adjust_read_range(iter->inode, folio, &block_start,
> block_end - block_start, &poff, &plen);
> --
> 2.40.1
>