linux-kernel - Re: [PATCH v7 6/8] block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Mon, 23 Jan 2023 11:28:40 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 6/8] block: Make bio structs pin pages rather than ref'ing if appropriate

In this function:

	void __bio_release_pages(struct bio *bio, bool mark_dirty)
	{
		unsigned int gup_flags = bio_to_gup_flags(bio);
		struct bvec_iter_all iter_all;
		struct bio_vec *bvec;

		bio_for_each_segment_all(bvec, bio, iter_all) {
			if (mark_dirty && !PageCompound(bvec->bv_page))
				set_page_dirty_lock(bvec->bv_page);
	>>>>		page_put_unpin(bvec->bv_page, gup_flags);
		}
	}

that ought to be a call to bio_release_page(), but the optimiser doesn't want
to inline it:-/

I found the only way I can get the compiler to properly inline it without it
repeating the calculations is to renumber the FOLL_* constants down and then
make bio_release_page() something like:

	static inline __attribute__((always_inline))
	void bio_release_page(struct bio *bio, struct page *page)
	{
		page_put_unpin(page,
		    ((bio->bi_flags & (1 << BIO_PAGE_REFFED)) ? FOLL_GET : 0) |
		    ((bio->bi_flags & (1 << BIO_PAGE_PINNED)) ? FOLL_PIN : 0));
	}

I guess the compiler optimiser isn't perfect yet:-)

David