linux-kernel - Re: [PATCH v5 8/9] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Message-ID: <15237.1673519321@warthog.procyon.org.uk>
Date:   Thu, 12 Jan 2023 10:28:41 +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 v5 8/9] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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

> 	if (cleanup_mode & FOLL_GET) {
> 		WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_PINNED));
> 		bio_set_flag(bio, BIO_PAGE_REFFED);
> 	}
> 	if (cleanup_mode & FOLL_PIN) {
> 		WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_REFFED));
> 		bio_set_flag(bio, BIO_PAGE_PINNED);
> 	}

That won't necessarily work as you might get back cleanup_mode == 0, in which
case both flags are cleared - and neither warning will trip on the next
addition.

I could change it so that rather than using a pair of flags, it uses a
four-state variable (which can be stored in bi_flags): BIO_PAGE_DEFAULT,
BIO_PAGE_REFFED, BIO_PAGE_PINNED, BIO_PAGE_NO_CLEANUP, say.

Or I could add an extra flag to say that the setting is locked.  Or we could
just live with the scenario I outlined possibly happening.

David