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

Darrick J. Wong
2021-07-15T22:55:43Z

On Thu, Jul 15, 2021 at 11:48:00PM +0100, Matthew Wilcox wrote:
> 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.

DOH. Yes, I just noticed that.

TBH I doubt anyone's really going to care about 4GB folios anyway.

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

--D