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

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Date:   Mon, 24 Aug 2020 03:20:26 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@...radead.org>,
        John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Ilya Dryomov <idryomov@...il.com>,
        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 8/23/20 12:25 AM, Christoph Hellwig wrote:
> 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.

(not relevant to this series as this patch has thankfully already been
dropped, just in general - but yes, definitely need a *strong* justification
to bump the bio size).

Would actually be nice to kill off a few flags, if possible, so the
flags space isn't totally full.

-- 
Jens Axboe