linux-kernel - Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO

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Message-ID: <Y89zypNE5z7rgdtX@infradead.org>
Date:   Mon, 23 Jan 2023 21:59:38 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        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 v8 04/10] iomap: don't get an reference on ZERO_PAGE for
 direct I/O block zeroing

On Mon, Jan 23, 2023 at 06:42:28PM -0800, John Hubbard wrote:
> > @@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
> >   	bio->bi_private = dio;
> >   	bio->bi_end_io = iomap_dio_bio_end_io;
> > -	get_page(page);
> > +	bio_set_flag(bio, BIO_NO_PAGE_REF);
> 
> ...is it accurate to assume that the entire bio is pointing to the zero
> page? I recall working through this area earlier last year, and ended up
> just letting the zero page get pinned, and then unpinning upon release,
> which is harmless.

Yes, the bio is built 4 lines above what is quoted here, and submitted
right after it.  It only contains the ZERO_PAGE.