linux-ext4 - Re: [RFCv3 6/7] iomap: Optimize iomap_read

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Date: Thu, 25 Apr 2024 23:53:04 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFCv3 6/7] iomap: Optimize iomap_read_folio

On Thu, Apr 25, 2024 at 06:58:50PM +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.

Maybe throw in a sentence here that this copies what
iomap_readahead_iter already does?

Otherwise this looks good to me modulo the offset comment from willy.