Re: [PATCH v6 09/34] bio: Rename BIO_NO_PAGE_REF to BIO_PAGE_REFFED and invert the meaning

[Christoph Hellwig]

On Wed, Jan 18, 2023 at 02:00:54PM +0000, David Howells wrote:
> Actually, should I make it so that the bottom two bits of bi_flags are a
> four-state variable and make it such that bio_release_page() gives a warning
> if the state is 0 - ie. unset?
> 
> The states would then be, say:
> 
> 	0	WARN(), do no cleanup
> 	1	FOLL_GET
> 	2	FOLL_PUT
> 	3	do no cleanup
> 
> This should help debug any places, such as iomap_dio_zero() that I just found,
> that add pages with refs without calling iov_iter_extract_pages().

I don't really see a point.  The fundamental use case of the bio itself
isn't really to this at all.  So we're stealing one, or in the future
two bits mostly to optimize some direct I/O use cases.  In fact I
wonder if instead we should just drop this micro-optimization entirely
an just add a member for the foll flags to the direct I/O container
structures (struct blkdev_dio, strut iomap_dio, struct dio, or just on
stack for __blkdev_direct_IO_simple and zonefs_file_dio_append) and
pass that to bio_release_pages.