linux-kernel - Re: [PATCH v6 09/34] bio: Rename BIO_NO_PAGE_REF to BIO_PAGE

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Date:   Wed, 18 Jan 2023 06:09:29 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 09/34] bio: Rename BIO_NO_PAGE_REF to BIO_PAGE_REFFED
 and invert the meaning

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.