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

From: David Howells <dhowells@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
	Matthew Wilcox <willy@infradead.org>,
	Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	Logan Gunthorpe <logang@deltatee.com>,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v7 6/8] block: Make bio structs pin pages rather than ref'ing if appropriate
Date: Mon, 23 Jan 2023 11:28:40 +0000	[thread overview]
Message-ID: <3813654.1674473320@warthog.procyon.org.uk> (raw)
In-Reply-To: <Y8vjlH+w1sNBPJjU@infradead.org>

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