linux-kernel - Re: [PATCH 4/5] bio: introduce BIO_FOLL

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Message-ID: <20200823062559.GA32480@infradead.org>
Date:   Sun, 23 Aug 2020 07:25:59 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] bio: introduce BIO_FOLL_PIN flag

On Fri, Aug 21, 2020 at 09:20:58PM -0700, John Hubbard wrote:
> Add a new BIO_FOLL_PIN flag to struct bio, whose "short int" flags field
> was full, thuse triggering an expansion of the field from 16, to 32
> bits. This allows for a nice assertion in bio_release_pages(), that the
> bio page release mechanism matches the page acquisition mechanism.
> 
> Set BIO_FOLL_PIN whenever pin_user_pages_fast() is used, and check for
> BIO_FOLL_PIN before using unpin_user_page().

When would the flag not be set when BIO_NO_PAGE_REF is not set?

Also I don't think we can't just expand the flags field, but I can send
a series to kill off two flags.