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

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Message-Id: <87wmokik3m.fsf@gmail.com>
Date: Fri, 26 Apr 2024 14:20:05 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Christoph Hellwig <hch@...radead.org>
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

Christoph Hellwig <hch@...radead.org> writes:

> 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?

Does this sound any better?

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.

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

Yes, I will address willy's comment too. 
Thanks for the review!

-ritesh