Re: [RFC PATCH 0/8] block: fix bio_add_XXX_page() return type

On Fri, May 21, 2021 at 12:30:45PM +0100, Matthew Wilcox wrote:

On Wed, May 19, 2021 at 11:22:47PM -0700, Chaitanya Kulkarni wrote:

quoted

The helper functions bio_add_XXX_page() returns the length which is
unsigned int but the return type of those functions is defined
as int instead of unsigned int.

I've been thinking about this for a few weeks as part of the folio
patches:

https://lore.kernel.org/linux-fsdevel/20210505150628.111735-72-willy@infradead.org/ (local)

 - len and off are measured in bytes
 - neither are permitted to be negative
 - for efficiency we only permit them to be up to 4GB

I therefore believe the correct type for these parameters to be size_t,
and we should range-check them if they're too large.  they should
actually always fit within the page that they're associated with, but
people do allocate non-compound pages and i'm not trying to break that
today.

using size_t makes it clear that these are byte counts, not (eg) sector
counts.  i do think it's good to make the return value unsigned so we
don't have people expecting a negative errno on failure.

I think the right type is bool.  We always return either 0 or the full
length we tried to add.  Instead of optimizing for a partial add (which
only makes sense for bio_add_hw_page anyway), I'd rather make the
interface as simple as possible.