Re: [PATCH 13/14] iomap: map multiple blocks at a time

Zhang Yi
2023-12-07T13:39:55Z

Hi, Christoph.

On 2023/12/7 15:27, Christoph Hellwig wrote:
> The ->map_blocks interface returns a valid range for writeback, but we
> still call back into it for every block, which is a bit inefficient.
>
> Change iomap_writepage_map to use the valid range in the map until the
> end of the folio or the dirty range inside the folio instead of calling
> back into every block.
>
> Note that the range is not used over folio boundaries as we need to be
> able to check the mapping sequence count under the folio lock.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/iomap/buffered-io.c | 116 ++++++++++++++++++++++++++++-------------
> include/linux/iomap.h | 7 +++
> 2 files changed, 88 insertions(+), 35 deletions(-)
>
[..]
> @@ -1738,29 +1775,41 @@ static int iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
>
> static int iomap_writepage_map_blocks(struct iomap_writepage_ctx *wpc,
> struct writeback_control *wbc, struct folio *folio,
> - struct inode *inode, u64 pos, unsigned *count)
> + struct inode *inode, u64 pos, unsigned dirty_len,
> + unsigned *count)
> {
> int error;
>
> - error = wpc->ops->map_blocks(wpc, inode, pos);
> - if (error)
> - goto fail;
> - trace_iomap_writepage_map(inode, &wpc->iomap);
> -
> - switch (wpc->iomap.type) {
> - case IOMAP_INLINE:
> - WARN_ON_ONCE(1);
> - error = -EIO;
> - break;
> - case IOMAP_HOLE:
> - break;
> - default:
> - error = iomap_add_to_ioend(wpc, wbc, folio, inode, pos);
> - if (!error)
> - (*count)++;
> - }
> + do {
> + unsigned map_len;
> +
> + error = wpc->ops->map_blocks(wpc, inode, pos);
> + if (error)
> + break;
> + trace_iomap_writepage_map(inode, &wpc->iomap);
> +
> + map_len = min_t(u64, dirty_len,
> + wpc->iomap.offset + wpc->iomap.length - pos);
> + WARN_ON_ONCE(!folio->private && map_len < dirty_len);

While I was debugging this series on ext4, I would suggest try to add map_len
or dirty_len into this trace point could be more convenient.

> +
> + switch (wpc->iomap.type) {
> + case IOMAP_INLINE:
> + WARN_ON_ONCE(1);
> + error = -EIO;
> + break;
> + case IOMAP_HOLE:
> + break;

BTW, I want to ask an unrelated question of this patch series. Do you
agree with me to add a IOMAP_DELAYED case and re-dirty folio here? The
background is that on ext4, jbd2 thread call ext4_normal_submit_inode_data_buffers()
submit data blocks in data=ordered mode, but it can only submit mapped
blocks, now we skip unmapped blocks and re-dirty folios in
ext4_do_writepages()->mpage_prepare_extent_to_map()->..->ext4_bio_write_folio().
So we have to inherit this logic when convert to iomap, I suppose ext4's
->map_blocks() return IOMAP_DELALLOC for this case, and iomap do something
like:

+ case IOMAP_DELALLOC:
+ iomap_set_range_dirty(folio, offset_in_folio(folio, pos),
+ map_len);
+ folio_redirty_for_writepage(wbc, folio);
+ break;

Thanks,
Yi.

> + default:
> + error = iomap_add_to_ioend(wpc, wbc, folio, inode, pos,
> + map_len);
> + if (!error)
> + (*count)++;
> + break;
> + }
> + dirty_len -= map_len;
> + pos += map_len;
> + } while (dirty_len && !error);
>
> -fail:
> /*
> * We cannot cancel the ioend directly here on error. We may have
> * already set other pages under writeback and hence we have to run I/O
> @@ -1827,7 +1876,7 @@ static bool iomap_writepage_handle_eof(struct folio *folio, struct inode *inode,
> * beyond i_size.
> */
> folio_zero_segment(folio, poff, folio_size(folio));
> - *end_pos = isize;
> + *end_pos = round_up(isize, i_blocksize(inode));
> }
>
> return true;