linux-kernel - Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Mon, 9 Jan 2023 14:57:36 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>, Jan Kara <jack@...e.cz>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather
 than ref'ing if appropriate

On 1/9/23 2:37?PM, David Howells wrote:
> Jan Kara <jack@...e.cz> wrote:
> 
>> So currently we already have BIO_NO_PAGE_REF flag and what you do in this
>> patch partially duplicates that. So either I'd drop that flag or instead of
>> bi_cleanup_mode variable (which honestly looks a bit wasteful given how we
>> microoptimize struct bio) just add another BIO_ flag...
> 
> I'm fine with translating the FOLL_* flags to the BIO_* flags.  I could add a
> BIO_PAGE_PINNED and translate:
> 
> 	FOLL_GET => 0
> 	FOLL_PIN => BIO_PAGE_PINNED
> 	0	 => BIO_NO_PAGE_REF
> 
> It would seem that BIO_NO_PAGE_REF can't be set for BIO_PAGE_PINNED because
> BIO_NO_PAGE_REF governs whether bio_release_pages() calls
> __bio_release_pages() - which would be necessary.  However, bio_release_page()
> can do one or the other on the basis of BIO_PAGE_PINNED being specified.  So
> in my patch I would end up with:
> 
> 	static void bio_release_page(struct bio *bio, struct page *page)
> 	{
> 		if (bio->bi_flags & BIO_NO_PAGE_REF)
> 			;
> 		else if (bio->bi_flags & BIO_PAGE_PINNED)
> 			unpin_user_page(page);
> 		else
> 			put_page(page);
> 	}

Let's please make this a bit more readable with:

static void bio_release_page(struct bio *bio, struct page *page)
{
	if (bio->bi_flags & BIO_NO_PAGE_REF)
		return;
	if (bio->bi_flags & BIO_PAGE_PINNED)
		unpin_user_page(page);
	else
		put_page(page);
}

-- 
Jens Axboe