linux-kernel - Re: [PATCH v14 098/138] iomap: Use folio offsets instead of page offsets

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Message-ID: <YPC7ILHEYv1JKKJW@casper.infradead.org>
Date:   Thu, 15 Jul 2021 23:48:00 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v14 098/138] iomap: Use folio offsets instead of page
 offsets

On Thu, Jul 15, 2021 at 02:26:57PM -0700, Darrick J. Wong wrote:
> > +	size_t poff = offset_in_folio(folio, *pos);
> > +	size_t plen = min_t(loff_t, folio_size(folio) - poff, length);
> 
> I'm confused about 'size_t poff' here vs. 'unsigned end' later -- why do
> we need a 64-bit quantity for poff?  I suppose some day we might want to
> have folios larger than 4GB or so, but so far we don't need that large
> of a byte offset within a page/folio, right?
> 
> Or are you merely moving the codebase towards using size_t for all byte
> offsets?

Both.  'end' isn't a byte count -- it's a block count.

> >  	if (orig_pos <= isize && orig_pos + length > isize) {
> > -		unsigned end = offset_in_page(isize - 1) >> block_bits;
> > +		unsigned end = offset_in_folio(folio, isize - 1) >> block_bits;

That right shift makes it not-a-byte-count.

I don't especially want to do all the work needed to support folios >2GB,
but I do like using size_t to represent a byte count.