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:

quoted

quoted

+	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.

quoted

quoted

 	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