linux-ext4 - Re: [PATCHv2 2/2] iomap: Optimize iomap_read

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Date: Tue, 7 May 2024 14:11:14 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCHv2 2/2] iomap: Optimize iomap_read_folio

On Tue, May 07, 2024 at 02:25:43PM +0530, Ritesh Harjani (IBM) wrote:
> iomap_readpage_iter() handles "uptodate blocks" and "not uptodate blocks"
> within a folio separately. This makes iomap_read_folio() to call into
> ->iomap_begin() to request for extent mapping even though it might already
> have an extent which is not fully processed.
> This happens when we either have a large folio or with bs < ps. In these
> cases we can have sub blocks which can be uptodate (say for e.g. due to
> previous writes). With iomap_read_folio_iter(), this is handled more
> efficiently by not calling ->iomap_begin() call until all the sub blocks
> with the current folio are processed.
> 
> iomap_read_folio_iter() handles multiple sub blocks within a given
> folio but it's implementation logic is similar to how
> iomap_readahead_iter() handles multiple folios within a single mapped
> extent. Both of them iterate over a given range of folio/mapped extent
> and call iomap_readpage_iter() for reading.
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> cc: Ojaswin Mujoo <ojaswin@...ux.ibm.com>

I like this improved changelog, it'e easier to understand why
_read_folio_iter needs to exist.

Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D

> ---
>  fs/iomap/buffered-io.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 9f79c82d1f73..a9bd74ee7870 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -444,6 +444,24 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
>  	return pos - orig_pos + plen;
>  }
> 
> +static loff_t iomap_read_folio_iter(const struct iomap_iter *iter,
> +		struct iomap_readpage_ctx *ctx)
> +{
> +	struct folio *folio = ctx->cur_folio;
> +	size_t offset = offset_in_folio(folio, iter->pos);
> +	loff_t length = min_t(loff_t, folio_size(folio) - offset,
> +			      iomap_length(iter));
> +	loff_t done, ret;
> +
> +	for (done = 0; done < length; done += ret) {
> +		ret = iomap_readpage_iter(iter, ctx, done);
> +		if (ret <= 0)
> +			return ret;
> +	}
> +
> +	return done;
> +}
> +
>  int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
>  {
>  	struct iomap_iter iter = {
> @@ -459,7 +477,7 @@ int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
>  	trace_iomap_readpage(iter.inode, 1);
> 
>  	while ((ret = iomap_iter(&iter, ops)) > 0)
> -		iter.processed = iomap_readpage_iter(&iter, &ctx, 0);
> +		iter.processed = iomap_read_folio_iter(&iter, &ctx);
> 
>  	if (ret < 0)
>  		folio_set_error(folio);
> --
> 2.44.0
> 
>