linux-kernel - Re: [PATCH v6 11/34] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Tue, 17 Jan 2023 08:26:08 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 11/34] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

Christoph Hellwig <hch@...radead.org> wrote:

> > +	bio_clear_flag(bio, BIO_PAGE_REFFED);
> 
> .. and this should not be needed.  Instead:
> 
> > +	cleanup_mode = iov_iter_extract_mode(iter, gup_flags);
> > +	if (cleanup_mode & FOLL_GET)
> > +		bio_set_flag(bio, BIO_PAGE_REFFED);
> > +	if (cleanup_mode & FOLL_PIN)
> > +		bio_set_flag(bio, BIO_PAGE_PINNED);
> 
> We could warn if a not match flag is set here if we really care.

Um... With these patches, BIO_PAGE_REFFED is set by default when the bio is
initialised otherwise every user of struct bio that currently adds pages
directly (assuming there are any) rather than going through
bio_iov_iter_get_pages() will have to set the flag, hence the need to clear
it.

Actually, I could do:

	if (!(cleanup_mode & FOLL_GET))
		bio_clear_flag(bio, BIO_PAGE_REFFED);
	if (cleanup_mode & FOLL_PIN)
		bio_set_flag(bio, BIO_PAGE_PINNED);

which should also work.

David